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/AvailabilityRepositoryTest.php

@@ -49,6 +49,101 @@ class AvailabilityRepositoryTest extends TestCase
         self::assertSame(42, $result);
     }
 
+    public function testInsertReturnsZeroWhenTheWriteFails(): void
+    {
+        Functions\expect('current_time')->with('mysql')->andReturn('2026-04-01 12:00:00');
+
+        // wpdb::insert returns false on error, but insert_id still holds the
+        // previous statement's id — returning it made a failed write look like a
+        // successful one.
+        $this->db->shouldReceive('insert')->once()->andReturn(false);
+        $this->db->insert_id = 42;
+
+        $slot = new AvailabilitySlot(5, '2026-04-01 09:00:00', '2026-04-01 10:00:00', 30);
+
+        self::assertSame(0, $this->repo->insert($slot));
+    }
+
+    public function testCreateFromWindowOmitsChunksThatFailedToInsert(): void
+    {
+        Functions\when('current_time')->justReturn('2026-04-01 12:00:00');
+
+        // Three chunks; the middle write fails.
+        $results = [null, false, null];
+        $ids     = [11, 13];
+
+        $this->db->shouldReceive('insert')
+            ->times(3)
+            ->andReturnUsing(function () use (&$results, &$ids) {
+                $outcome = array_shift($results);
+
+                if (false !== $outcome) {
+                    $this->db->insert_id = array_shift($ids);
+                }
+
+                return $outcome;
+            });
+
+        $window = new AvailabilitySlot(5, '2026-04-01 09:00:00', '2026-04-01 10:30:00', 30);
+
+        self::assertSame([11, 13], $this->repo->createFromWindow($window));
+    }
+
+    public function testWeeklySeriesIsClampedToTheMaximum(): void
+    {
+        Functions\when('current_time')->justReturn('2026-04-01 12:00:00');
+
+        // The form's max is advisory; a hand-crafted POST could ask for any
+        // number, so the ceiling is enforced here.
+        $next = 1;
+        $this->db->shouldReceive('insert')
+            ->times(AvailabilitySlot::MAX_WEEKLY_OCCURRENCES)
+            ->andReturnUsing(function () use (&$next) {
+                $this->db->insert_id = $next++;
+
+                return null;
+            });
+        $this->db->shouldReceive('update')->once();
+
+        $first = new AvailabilitySlot(5, '2026-04-01 09:00:00', '2026-04-01 10:00:00', 60);
+
+        self::assertCount(
+            AvailabilitySlot::MAX_WEEKLY_OCCURRENCES,
+            $this->repo->createWeeklySeries($first, 10000)
+        );
+    }
+
+    public function testWeeklySeriesGroupsOnTheFirstRowThatActuallyWrote(): void
+    {
+        Functions\when('current_time')->justReturn('2026-04-01 12:00:00');
+
+        // The first insert fails. A failed row must not become the recurrence
+        // group (its id is 0), which would orphan every later occurrence.
+        $results = [false, null, null];
+        $ids     = [21, 22];
+
+        $this->db->shouldReceive('insert')
+            ->times(3)
+            ->andReturnUsing(function () use (&$results, &$ids) {
+                $outcome = array_shift($results);
+
+                if (false !== $outcome) {
+                    $this->db->insert_id = array_shift($ids);
+                }
+
+                return $outcome;
+            });
+
+        // The group is set from row 21 — the first that survived.
+        $this->db->shouldReceive('update')
+            ->once()
+            ->with('wp_us_availability', ['recurrence_group' => 21], ['id' => 21], ['%d'], ['%d']);
+
+        $first = new AvailabilitySlot(5, '2026-04-01 09:00:00', '2026-04-01 10:00:00', 60);
+
+        self::assertSame([21, 22], $this->repo->createWeeklySeries($first, 3));
+    }
+
     public function testCreateWeeklySeriesInsertsWeeklyAndSharesGroup(): void
     {
         Functions\when('current_time')->justReturn('2026-04-07 12:00:00');