Stop the availability form failing in silence

Adding availability for 5:30-6:00 PM with the lesson length left on its
60-minute default saved nothing and said nothing. A window is stored as
consecutive lesson-length slots, so one that fits no lesson splits into
none: splitByDuration() returned [], createFromWindow() inserted
nothing, and addSlot() discarded the result and re-rendered the page
unchanged.

The REST endpoint already rejected that window with a 400. The admin
form checked the same rules separately, and its copy was both laxer and
mute — an unreadable date, an end before the start, and a two-day window
were bare `return`s, and it never checked offering ownership at all, so
a crafted POST could tie a slot to another instructor's offering and
inherit their price and payment routing.

Both callers now go through WindowValidator, which returns the window or
a WP_Error explaining the refusal. The endpoint returns that error as
is; the page renders its message as a notice. handleFormAction returns
a [notice, error] pair so deletes report themselves too, and a
successful add says how many slots it created.

Two failures could also go unnoticed underneath: wpdb::insert's result
was ignored, and insert_id still holds the previous statement's id after
a failed write, so a failure looked like a success — and could become
the recurrence group of a weekly series, orphaning every later
occurrence. weeks was unbounded server-side despite the form's max=52.

availability-admin.js narrows the lesson-length choices to those that
fit the window and blocks submission when none do, which is what makes
the original mistake hard to repeat. It is a convenience: the server
validates regardless.

Closes #130

tests/Unit/Availability/AvailabilityControllerTest.php

@@ -8,6 +8,7 @@ use Mockery;
 use Unsupervised\Schedular\Availability\AvailabilityController;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Availability\AvailabilitySlot;
+use Unsupervised\Schedular\Availability\WindowValidator;
 use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Tests\Unit\TestCase;
 
@@ -23,7 +24,7 @@ class AvailabilityControllerTest extends TestCase
 
         $this->repository = Mockery::mock(AvailabilityRepository::class);
         $this->offerings  = Mockery::mock(OfferingRepository::class);
-        $this->controller = new AvailabilityController($this->repository, $this->offerings);
+        $this->controller = new AvailabilityController($this->repository, $this->offerings, new WindowValidator($this->offerings));
 
         $_POST = [];
         $_GET  = [];
@@ -39,6 +40,7 @@ class AvailabilityControllerTest extends TestCase
         Functions\when('absint')->alias(static fn ($value) => abs((int) $value));
         Functions\when('get_option')->justReturn(1);
         Functions\when('current_time')->justReturn('2026-07-06');
+        Functions\when('selected')->justReturn('');
         Functions\when('admin_url')->justReturn('admin.php?page=us-availability');
         Functions\when('add_query_arg')->justReturn('admin.php?page=us-availability&usc_view=week');
         Functions\when('wp_nonce_field')->justReturn('');
@@ -131,6 +133,159 @@ class AvailabilityControllerTest extends TestCase
         self::assertStringNotContainsString('name="slot_ids[]" form="usc-bulk-delete-form" value="6"', $html);
     }
 
+    /**
+     * The reported bug: a 30-minute window submitted with the lesson length left
+     * on 60 saved nothing and said nothing. It must now say why.
+     */
+    public function testAddShowsAnErrorWhenTheWindowIsShorterThanTheLessonLength(): void
+    {
+        $_POST = [
+            'usc_action'       => 'add',
+            'start_dt'         => '2026-09-10T17:30',
+            'end_dt'           => '2026-09-10T18:00',
+            'duration_minutes' => '60',
+        ];
+
+        $this->repository->shouldNotReceive('createFromWindow');
+        $this->repository->shouldReceive('findByInstructor')->once()->with(3)->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('notice-error', $html);
+        self::assertStringContainsString('60-minute lesson length', $html);
+        self::assertStringNotContainsString('notice-success', $html);
+    }
+
+    /** @return array<string, array{array<string, string>, string}> */
+    public static function invalidWindows(): array
+    {
+        return [
+            'unparseable start' => [['start_dt' => 'whenever', 'end_dt' => '2026-09-10T18:00'], 'valid start and end'],
+            'end before start'  => [['start_dt' => '2026-09-10T18:00', 'end_dt' => '2026-09-10T17:00'], 'after the start time'],
+            'spans two days'    => [['start_dt' => '2026-09-10T23:00', 'end_dt' => '2026-09-11T01:00'], 'same day'],
+        ];
+    }
+
+    /**
+     * Each of these used to be a bare `return` — the page reloaded unchanged and
+     * the instructor had no way to tell the save had failed.
+     *
+     * @dataProvider invalidWindows
+     *
+     * @param array<string, string> $fields
+     */
+    public function testAddReportsEveryRejectedWindow(array $fields, string $expected): void
+    {
+        $_POST = array_merge([ 'usc_action' => 'add', 'duration_minutes' => '30' ], $fields);
+
+        $this->repository->shouldNotReceive('createFromWindow');
+        $this->repository->shouldReceive('findByInstructor')->once()->with(3)->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('notice-error', $html);
+        self::assertStringContainsString($expected, $html);
+    }
+
+    public function testAddRejectsAnOfferingTheInstructorDoesNotOwn(): void
+    {
+        // The REST endpoint always checked this; the admin form never did, so a
+        // crafted POST could tie a slot to another instructor's offering.
+        $_POST = [
+            'usc_action'       => 'add',
+            'start_dt'         => '2026-09-10T17:00',
+            'end_dt'           => '2026-09-10T18:00',
+            'duration_minutes' => '60',
+            'offering_id'      => '8',
+        ];
+
+        $this->offerings->shouldReceive('findById')->once()->with(8)->andReturn(null);
+        $this->repository->shouldNotReceive('createFromWindow');
+        $this->repository->shouldReceive('findByInstructor')->once()->with(3)->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('notice-error', $html);
+        self::assertStringContainsString('not available', $html);
+    }
+
+    public function testAddReportsHowManySlotsWereCreated(): void
+    {
+        $_POST = [
+            'usc_action'       => 'add',
+            'start_dt'         => '2026-09-10T17:00',
+            'end_dt'           => '2026-09-10T19:00',
+            'duration_minutes' => '60',
+            'recurrence'       => 'weekly',
+            'weeks'            => '41',
+        ];
+
+        $this->repository->shouldReceive('createFromWindow')->once()
+            ->with(Mockery::type(AvailabilitySlot::class), true, 41)
+            ->andReturn(range(1, 82));
+        $this->repository->shouldReceive('findByInstructor')->once()->with(3)->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('notice-success', $html);
+        self::assertStringContainsString('Added 82 bookable slots.', $html);
+    }
+
+    public function testAddReportsAFailedWrite(): void
+    {
+        // A valid window always splits into at least one slot, so an empty result
+        // means the inserts themselves failed.
+        $_POST = [
+            'usc_action'       => 'add',
+            'start_dt'         => '2026-09-10T17:00',
+            'end_dt'           => '2026-09-10T18:00',
+            'duration_minutes' => '60',
+        ];
+
+        $this->repository->shouldReceive('createFromWindow')->once()->andReturn([]);
+        $this->repository->shouldReceive('findByInstructor')->once()->with(3)->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('notice-error', $html);
+        self::assertStringContainsString('could not be saved', $html);
+    }
+
+    public function testSingleDeleteReportsAFailureToDelete(): void
+    {
+        $_POST = [ 'usc_action' => 'delete', 'slot_id' => '7' ];
+
+        $other = new AvailabilitySlot(instructorId: 4, startDt: '2026-07-08 09:00:00', endDt: '2026-07-08 10:00:00', id: 7);
+
+        $this->repository->shouldReceive('findById')->once()->with(7)->andReturn($other);
+        $this->repository->shouldReceive('findByInstructor')->once()->with(3)->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('notice-error', $html);
+        self::assertStringContainsString('could not be deleted', $html);
+    }
+
+    public function testBulkDeleteReportsBothHalvesOfAPartialResult(): void
+    {
+        $_POST = [ 'usc_action' => 'bulk_delete', 'slot_ids' => ['5', '7'] ];
+
+        $owned  = new AvailabilitySlot(instructorId: 3, startDt: '2026-07-08 09:00:00', endDt: '2026-07-08 10:00:00', id: 5);
+        $booked = new AvailabilitySlot(instructorId: 3, startDt: '2026-07-08 10:00:00', endDt: '2026-07-08 11:00:00', isBooked: true, id: 7);
+
+        $this->repository->shouldReceive('findById')->once()->with(5)->andReturn($owned);
+        $this->repository->shouldReceive('findById')->once()->with(7)->andReturn($booked);
+        $this->repository->shouldReceive('delete')->once()->with(5)->andReturn(true);
+        // The repository refuses a booked row, reporting it by returning false.
+        $this->repository->shouldReceive('delete')->once()->with(7)->andReturn(false);
+        $this->repository->shouldReceive('findByInstructor')->once()->with(3)->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('1 slot deleted.', $html);
+        self::assertStringContainsString('1 slot could not be deleted', $html);
+    }
+
     private function render(): string
     {
         ob_start();