Skip payment step for unpriced bookings, confirm them immediately, show students their lessons

Booking a slot with no priced offering created the lesson but no payment,
yet the front end still called POST /payments/intent, which 400ed with
"Could not start payment for this registration" — the student saw an error
while the backend held a claimed slot and a lesson stuck at pending.

- POST /bookings and POST /enrollments now return a `payment` summary
  ({id, method, status}) or null when nothing is owed; the JS only runs
  the payment step when a payment exists.
- Bookings with nothing owed are confirmed at creation — there is no
  payment step that would ever confirm them later.
- The booking page now shows the student's upcoming lessons (GET /bookings,
  now scoped to upcoming non-cancelled lessons with slot start/end times)
  with a pending-payment/confirmed status badge.

Fixes #53

Co-Authored-By: Claude Fable 5 <[email protected]>

tests/Unit/GroupClass/EnrollmentEndpointTest.php

@@ -0,0 +1,101 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\GroupClass;
+
+use Brain\Monkey\Functions;
+use Mockery;
+use Unsupervised\Schedular\GroupClass\EnrollmentEndpoint;
+use Unsupervised\Schedular\GroupClass\EnrollmentRepository;
+use Unsupervised\Schedular\Offering\Offering;
+use Unsupervised\Schedular\Offering\OfferingRepository;
+use Unsupervised\Schedular\Payment\Payment;
+use Unsupervised\Schedular\Payment\PaymentService;
+use Unsupervised\Schedular\Registration\RegistrationGate;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class EnrollmentEndpointTest extends TestCase
+{
+    private EnrollmentRepository $enrollments;
+    private OfferingRepository $offerings;
+    private RegistrationGate $gate;
+    private PaymentService $payments;
+    private EnrollmentEndpoint $endpoint;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        Functions\when('absint')->alias(static fn ($v): int => abs((int) $v));
+        Functions\when('wp_unslash')->returnArg();
+        Functions\when('sanitize_text_field')->returnArg();
+        Functions\when('get_current_user_id')->justReturn(5);
+
+        $this->enrollments = Mockery::mock(EnrollmentRepository::class);
+        $this->offerings   = Mockery::mock(OfferingRepository::class);
+        $this->gate        = Mockery::mock(RegistrationGate::class);
+        $this->payments    = Mockery::mock(PaymentService::class);
+
+        $this->endpoint = new EnrollmentEndpoint(
+            $this->enrollments,
+            $this->offerings,
+            $this->gate,
+            $this->payments,
+        );
+    }
+
+    private function offering(float $price): Offering
+    {
+        return new Offering(instructorId: 3, kind: Offering::KIND_GROUP_CLASS, title: 'Choir', price: $price, id: 8);
+    }
+
+    private function expectSuccessfulEnrollment(): void
+    {
+        $this->enrollments->shouldReceive('hasActiveEnrollment')->with(8, 5)->andReturn(false);
+        $this->enrollments->shouldReceive('countActiveForOffering')->never();
+        $this->gate->shouldReceive('validate')->andReturn(null);
+        $this->enrollments->shouldReceive('insert')->once()->andReturn(44);
+        $this->gate->shouldReceive('record')->once();
+    }
+
+    public function testEnrollInFreeClassReturnsNullPayment(): void
+    {
+        $this->offerings->shouldReceive('findById')->with(8)->andReturn($this->offering(0.0));
+        $this->expectSuccessfulEnrollment();
+        $this->payments->shouldNotReceive('createForRegistration');
+
+        $result = $this->endpoint->enroll(new \WP_REST_Request(['offering_id' => 8]));
+
+        self::assertInstanceOf(\WP_REST_Response::class, $result);
+        self::assertSame(201, $result->get_status());
+        self::assertSame(44, $result->get_data()['id']);
+        self::assertNull($result->get_data()['payment']);
+    }
+
+    public function testEnrollInPricedClassReturnsPaymentSummary(): void
+    {
+        $this->offerings->shouldReceive('findById')->with(8)->andReturn($this->offering(120.0));
+        $this->expectSuccessfulEnrollment();
+        $this->payments->shouldReceive('createForRegistration')
+            ->once()
+            ->with(Payment::REG_ENROLLMENT, 44, 5, 3, 120.0, 'CAD', null)
+            ->andReturn(new Payment(
+                studentId:        5,
+                instructorId:     3,
+                registrationType: Payment::REG_ENROLLMENT,
+                registrationId:   44,
+                amount:           120.0,
+                method:           Payment::METHOD_ETRANSFER,
+                status:           Payment::STATUS_PENDING,
+                id:               12,
+            ));
+
+        $result = $this->endpoint->enroll(new \WP_REST_Request(['offering_id' => 8]));
+
+        self::assertInstanceOf(\WP_REST_Response::class, $result);
+        self::assertSame(
+            ['id' => 12, 'method' => Payment::METHOD_ETRANSFER, 'status' => Payment::STATUS_PENDING],
+            $result->get_data()['payment']
+        );
+    }
+}