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/FormBuilder/Bindings/FormBindingExceptionClassifierTest.php

@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\FormBuilder\Bindings;
+
+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 App\FormBuilder\Bindings\FormBindingExceptionClassifier;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ *
+ * The classifier is a single-responsibility helper: Throwable in,
+ * failure_response_code string out. Centralised so the listener catch
+ * block (D2) and the retry-service recordFailure (D2) produce identical
+ * classifications.
+ */
+final class FormBindingExceptionClassifierTest extends TestCase
+{
+    public function test_classifies_schema_config_exception(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingSchemaConfigException(submissionId: '01HX', message: 'x'),
+        );
+        $this->assertSame('schema_config_error', $code);
+    }
+
+    public function test_classifies_infra_exception(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingInfraException(submissionId: '01HX', message: 'x'),
+        );
+        $this->assertSame('temporary_error', $code);
+    }
+
+    public function test_classifies_timeout_as_temporary_error(): void
+    {
+        // Subclass dispatch — Timeout extends Infra, inherits reasonCode.
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingApplicatorTimeoutException(submissionId: '01HX', message: 'deadline'),
+        );
+        $this->assertSame('temporary_error', $code);
+    }
+
+    public function test_classifies_data_integrity_exception(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingDataIntegrityException(submissionId: '01HX', message: 'fk'),
+        );
+        $this->assertSame('data_integrity_error', $code);
+    }
+
+    public function test_classifies_arbitrary_runtime_exception_as_unknown(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(new RuntimeException('boom'));
+        $this->assertSame('unknown_error', $code);
+    }
+
+    public function test_classifies_identity_match_invariant_violation_as_unknown(): void
+    {
+        // IdentityMatchInvariantViolation is intentionally NOT in the
+        // FormBindingApplicatorException hierarchy (per RFC-WS-6 §Q2 — the
+        // listener that throws it runs outside the binding-applicator
+        // pipeline). It falls through to 'unknown_error' here, which is
+        // the right response-shape because users cannot meaningfully act
+        // on it; admins triage via GlitchTip.
+        $code = FormBindingExceptionClassifier::classify(new IdentityMatchInvariantViolation('x'));
+        $this->assertSame('unknown_error', $code);
+    }
+}