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/Schema/Ws6V13DeltaD1MigrationTest.php

@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\FormBuilder\Schema;
+
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+use Tests\TestCase;
+
+/**
+ * Migration rehearsal for WS-6 v1.3-delta D1:
+ *   - 2026_05_08_000001_add_failure_response_code_to_form_submissions
+ *
+ * Verifies the column + index land per RFC-WS-6 §Q3 v1.3 addition 2 and
+ * ARCH-BINDINGS §7.1 v1.2.
+ */
+final class Ws6V13DeltaD1MigrationTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_form_submissions_has_failure_response_code_column(): void
+    {
+        $this->assertTrue(Schema::hasColumn('form_submissions', 'failure_response_code'));
+    }
+
+    public function test_failure_response_code_is_nullable_string_40(): void
+    {
+        $row = DB::selectOne(
+            'SELECT DATA_TYPE, CHARACTER_MAXIMUM_LENGTH, IS_NULLABLE, COLUMN_DEFAULT
+             FROM information_schema.COLUMNS
+             WHERE TABLE_SCHEMA = DATABASE()
+               AND TABLE_NAME = ?
+               AND COLUMN_NAME = ?',
+            ['form_submissions', 'failure_response_code'],
+        );
+
+        $this->assertNotNull($row, 'failure_response_code column missing');
+        $this->assertSame('varchar', strtolower((string) $row->DATA_TYPE));
+        $this->assertSame(40, (int) $row->CHARACTER_MAXIMUM_LENGTH);
+        $this->assertSame('YES', $row->IS_NULLABLE);
+        $this->assertNull($row->COLUMN_DEFAULT);
+    }
+
+    public function test_failure_response_code_index_present(): void
+    {
+        $row = DB::selectOne(
+            'SELECT INDEX_NAME
+             FROM information_schema.STATISTICS
+             WHERE TABLE_SCHEMA = DATABASE()
+               AND TABLE_NAME = ?
+               AND INDEX_NAME = ?',
+            ['form_submissions', 'fs_failure_response_code_idx'],
+        );
+
+        $this->assertNotNull($row, 'fs_failure_response_code_idx missing');
+    }
+}