feat(form-builder): FormSubmission cast + factory state for failure_response_code

Per RFC-WS-6 §Q3 v1.3 addition 2.

- Added 'failure_response_code' to FormSubmission $fillable + 'string' cast.
  Plain string (not enum) — the exception subclass on
  form_submission_action_failures is the canonical classification source;
  this column is a denormalised mirror for response-shape rendering.
- Factory fluent state method withFailureResponseCode() with documentation
  of the four valid values.

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

api/database/factories/FormBuilder/FormSubmissionFactory.php

@@ -64,4 +64,23 @@ final class FormSubmissionFactory extends Factory
             'organisation_id' => $event->organisation_id,
         ]);
     }
+
+    /**
+     * Set failure_response_code. Typically used in tests asserting the
+     * downstream rendering behaviour for failed submissions.
+     *
+     * Valid values per RFC-WS-6 §Q3 v1.3 addition 2:
+     * - 'schema_config_error'
+     * - 'temporary_error'
+     * - 'data_integrity_error'
+     * - 'unknown_error'
+     *
+     * Note: writing this value via factory does NOT itself set apply_status
+     * to FAILED. Pair with an explicit apply_status attribute in tests
+     * that need a coherent failed-submission fixture.
+     */
+    public function withFailureResponseCode(string $code): static
+    {
+        return $this->state(fn () => ['failure_response_code' => $code]);
+    }
 }