Let the studio register the students who cannot register themselves

The Book a lesson for a student panel built its picker from the us_student
role but vetted the submission with the book_lesson capability. ChildLoginGate
and RegistrationLoginGate withhold that capability from accounts that keep the
role, so the panel offered every guardian-managed child and every unapproved
signup and then refused them — with a message claiming no student had been
chosen, and a form cleared of all five fields.

Withholding book_lesson stops those accounts registering in their own name. It
was never meant to stop the studio acting for them, which is what the panel is
for, and for a child is the only route to a lesson besides their guardian.

Guard the student role instead, via a new RoleManager::isStudent() shared with
every picker and guard on the staff side so the two cannot drift apart again.
Group enrolment gets the same predicate: addDirect() and grantAccess() vetted
their posted ids not at all, and would enrol an instructor, an administrator,
or an account deleted since the page was drawn — raising a real payment against
them for a priced class.

Keep a refused booking's fields as submitted, reading the form through one
LessonController::submittedBooking() so what gets booked and what is shown
again cannot disagree about a field name. A booking that succeeds still leaves
an empty form, so the next one does not inherit it.

Closes #185

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01XunBYk2sFEc1oL14sUiuBU

tests/Unit/Booking/LessonControllerTest.php

@@ -316,6 +316,54 @@ class LessonControllerTest extends TestCase
         self::assertStringContainsString('name="usc_action" value="book_for_student"', $html);
     }
 
+    /**
+     * A refusal used to clear all five fields, so one mistake meant retyping the
+     * whole form — and the panel is only ever reopened *because* something was
+     * refused.
+     */
+    public function testARefusedBookingComesBackWithEveryFieldStillFilledIn(): void
+    {
+        $this->postBooking();
+        $this->offerPanel();
+
+        $this->adminBooking->shouldReceive('book')->once()->andReturn(
+            new \WP_Error('slot_taken', 'That time has already been booked.')
+        );
+        $this->bookings->shouldReceive('findAllUpcoming')->once()->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('That time has already been booked.', $html);
+        // Attribute spacing in the template is not what is under test here.
+        $tags = (string) preg_replace('/\s+/', ' ', $html);
+
+        // The panel is reopened, showing the student, time and lesson type as posted.
+        self::assertStringContainsString(' open>', $html);
+        self::assertStringContainsString('value="42" selected=\'selected\'', $tags);
+        self::assertStringContainsString('value="7" selected=\'selected\'', $tags);
+        self::assertStringContainsString('value="3" selected=\'selected\'', $tags);
+        // Both ticks and the note survive too.
+        self::assertSame(2, substr_count($html, "checked='checked'"));
+        self::assertStringContainsString('value="Make-up lesson"', $html);
+    }
+
+    public function testASuccessfulBookingLeavesAnEmptyFormForTheNextOne(): void
+    {
+        $this->postBooking();
+        $this->offerPanel();
+
+        $this->adminBooking->shouldReceive('book')->once()->andReturn('Booked.');
+        $this->bookings->shouldReceive('findAllUpcoming')->once()->andReturn([]);
+
+        $html = $this->render();
+
+        self::assertStringContainsString('Booked.', $html);
+        // Nothing carried over, or the next booking silently inherits this one's.
+        self::assertStringNotContainsString("selected='selected'", $html);
+        self::assertStringNotContainsString("checked='checked'", $html);
+        self::assertStringContainsString('value=""', $html);
+    }
+
     public function testTheStudioSchedulerBooksAgainstAnyInstructorsTime(): void
     {
         $this->postBooking();
@@ -390,6 +438,16 @@ class LessonControllerTest extends TestCase
         Functions\when('absint')->alias(static fn ($v): int => abs((int) $v));
     }
 
+    /** The panel with one of each choice, so a re-selected value has somewhere to land. */
+    private function offerPanel(): void
+    {
+        $this->adminBooking->shouldReceive('formData')->once()->andReturn([
+            'students'  => [['id' => 42, 'name' => 'Ada Lovelace']],
+            'offerings' => [['id' => 3, 'label' => '30 min piano (30 min)']],
+            'slots'     => [['id' => 7, 'label' => 'Wed Jul 1, 2026 10:00 AM (30 min)', 'weekly' => false]],
+        ]);
+    }
+
     public function testAStaffBookedLessonOffersTheRecordIntakeForm(): void
     {
         $_GET['lesson_id'] = '1';