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/Feature/FormBuilder/FormSubmissionFailureResponseCodeTest.php

@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\FormBuilder;
+
+use App\Enums\FormBuilder\ApplyStatus;
+use App\Models\FormBuilder\FormSubmission;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ *
+ * Verifies the cast (plain string), default-NULL behaviour, and that
+ * the factory state method composes with apply_status attribute writes
+ * for coherent failed-submission fixtures.
+ */
+final class FormSubmissionFailureResponseCodeTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_failure_response_code_persists_as_string(): void
+    {
+        $submission = FormSubmission::factory()
+            ->withFailureResponseCode('schema_config_error')
+            ->create();
+
+        $reloaded = $submission->fresh();
+        $this->assertSame('schema_config_error', $reloaded->failure_response_code);
+        $this->assertIsString($reloaded->failure_response_code);
+    }
+
+    public function test_failure_response_code_null_by_default(): void
+    {
+        $submission = FormSubmission::factory()->create();
+        $this->assertNull($submission->fresh()->failure_response_code);
+    }
+
+    public function test_factory_state_composes_with_apply_status(): void
+    {
+        $submission = FormSubmission::factory()
+            ->withFailureResponseCode('temporary_error')
+            ->create(['apply_status' => ApplyStatus::FAILED]);
+
+        $reloaded = $submission->fresh();
+        $this->assertSame(ApplyStatus::FAILED, $reloaded->apply_status);
+        $this->assertSame('temporary_error', $reloaded->failure_response_code);
+    }
+
+    public function test_failure_response_code_round_trips_for_each_canonical_value(): void
+    {
+        foreach (['schema_config_error', 'temporary_error', 'data_integrity_error', 'unknown_error'] as $code) {
+            $submission = FormSubmission::factory()
+                ->withFailureResponseCode($code)
+                ->create();
+
+            $this->assertSame($code, $submission->fresh()->failure_response_code);
+        }
+    }
+}