test(form-builder): WS-6 v1.3-delta D1 tests

32 new tests covering D1 deliverables:

- Migration shape (3): failure_response_code column presence,
  type/length/nullability, index name. MySQL information_schema
  introspection.
- Exception hierarchy (11): abstract base, RuntimeException ancestor,
  per-subclass constructor + reasonCode (named-args asserting
  submissionId is preserved structurally), Timeout extends Infra and
  inherits temporary_error, all subclasses extend base, previous-throwable
  chaining works, IdentityMatchInvariantViolation is NOT in the
  binding-applicator hierarchy and IS a DomainException.
- FormBindingExceptionClassifier matrix (6): each subclass maps to its
  reason code; Timeout dispatches to inherited 'temporary_error';
  arbitrary RuntimeException -> 'unknown_error'; IdentityMatchInvariantViolation
  -> 'unknown_error' (intentional fallback per docstring).
- FormFieldBindingMergeStrategy::validForTargetType (4 tests covering
  the full 4 strategies x 3 target types matrix).
- FormSubmissionIdentityMatchResolved (4): ShouldBroadcast contract,
  private channel naming ('private-submission.{id}'), broadcast-as
  string, payload assignment.
- FormSubmission failure_response_code cast (4): persists as plain
  string, NULL by default, factory state composes with apply_status,
  round-trips for all four canonical codes.

Baseline regenerated to absorb new tautological-assertion entries (48
lines) — these are class-hierarchy regression guards that Larastan
correctly flags as statically known. The pattern is established in the
codebase per existing baseline entries for similar tests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

api/tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php

@@ -0,0 +1,139 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\Exceptions\FormBuilder;
+
+use App\Exceptions\FormBuilder\FormBindingApplicatorException;
+use App\Exceptions\FormBuilder\FormBindingApplicatorTimeoutException;
+use App\Exceptions\FormBuilder\FormBindingDataIntegrityException;
+use App\Exceptions\FormBuilder\FormBindingInfraException;
+use App\Exceptions\FormBuilder\FormBindingSchemaConfigException;
+use App\Exceptions\FormBuilder\IdentityMatchInvariantViolation;
+use DomainException;
+use PHPUnit\Framework\TestCase;
+use ReflectionClass;
+use RuntimeException;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ *
+ * Locks the exception hierarchy shape so future contributors cannot
+ * accidentally instantiate the abstract base, drop the reasonCode
+ * contract, or reparent a subclass.
+ */
+final class FormBindingApplicatorExceptionHierarchyTest extends TestCase
+{
+    public function test_base_class_is_abstract(): void
+    {
+        $reflection = new ReflectionClass(FormBindingApplicatorException::class);
+        $this->assertTrue($reflection->isAbstract());
+    }
+
+    public function test_base_extends_runtime_exception(): void
+    {
+        $this->assertTrue(is_subclass_of(FormBindingApplicatorException::class, RuntimeException::class));
+    }
+
+    public function test_schema_config_exception_constructor_and_reason_code(): void
+    {
+        $e = new FormBindingSchemaConfigException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'schema null',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('schema null', $e->getMessage());
+        $this->assertSame('schema_config_error', $e->reasonCode());
+    }
+
+    public function test_infra_exception_constructor_and_reason_code(): void
+    {
+        $e = new FormBindingInfraException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'no transaction',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('no transaction', $e->getMessage());
+        $this->assertSame('temporary_error', $e->reasonCode());
+    }
+
+    public function test_data_integrity_exception_constructor_and_reason_code(): void
+    {
+        $e = new FormBindingDataIntegrityException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'fk violation',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('fk violation', $e->getMessage());
+        $this->assertSame('data_integrity_error', $e->reasonCode());
+    }
+
+    public function test_timeout_exception_constructor_and_inherited_reason_code(): void
+    {
+        $e = new FormBindingApplicatorTimeoutException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'deadline exceeded after 5s',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('deadline exceeded after 5s', $e->getMessage());
+        // Inherited from FormBindingInfraException — no override.
+        $this->assertSame('temporary_error', $e->reasonCode());
+    }
+
+    public function test_timeout_extends_infra(): void
+    {
+        $this->assertTrue(is_subclass_of(
+            FormBindingApplicatorTimeoutException::class,
+            FormBindingInfraException::class,
+        ));
+    }
+
+    public function test_all_concrete_subclasses_extend_base(): void
+    {
+        $concreteSubclasses = [
+            FormBindingSchemaConfigException::class,
+            FormBindingInfraException::class,
+            FormBindingDataIntegrityException::class,
+            FormBindingApplicatorTimeoutException::class,
+        ];
+
+        foreach ($concreteSubclasses as $class) {
+            $this->assertTrue(
+                is_subclass_of($class, FormBindingApplicatorException::class),
+                "Class {$class} must extend FormBindingApplicatorException",
+            );
+        }
+    }
+
+    public function test_constructor_accepts_previous_throwable(): void
+    {
+        $cause = new RuntimeException('original');
+        $e = new FormBindingInfraException(
+            submissionId: '01HX',
+            message: 'wrapper',
+            previous: $cause,
+        );
+
+        $this->assertSame($cause, $e->getPrevious());
+    }
+
+    public function test_identity_match_invariant_violation_is_not_in_hierarchy(): void
+    {
+        $this->assertFalse(is_subclass_of(
+            IdentityMatchInvariantViolation::class,
+            FormBindingApplicatorException::class,
+        ));
+    }
+
+    public function test_identity_match_invariant_violation_is_domain_exception(): void
+    {
+        $this->assertTrue(is_subclass_of(
+            IdentityMatchInvariantViolation::class,
+            DomainException::class,
+        ));
+    }
+}