feat(form-builder): FormBindingExceptionClassifier helper

Per RFC-WS-6 §Q3 v1.3 addition 2.

Centralises the Throwable -> failure_response_code mapping so the
listener (ApplyBindingsOnFormSubmit::handle catch block) and the
retry-service (FormFailureRetryService::recordFailure) produce
identical classifications. Single behaviour-change point.

Resolution order: FormBindingApplicatorException subclass dispatch via
reasonCode(); fallback 'unknown_error' for anything outside the hierarchy.

Wiring into the listener and the retry service lands in D2.

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

api/app/FormBuilder/Bindings/FormBindingExceptionClassifier.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Bindings;
+
+use App\Exceptions\FormBuilder\FormBindingApplicatorException;
+use Throwable;
+
+/**
+ * Maps a Throwable to a failure_response_code string.
+ *
+ * Used by both ApplyBindingsOnFormSubmit::handle's catch block (D2) and
+ * FormFailureRetryService::recordFailure (D2 update). Centralised so the
+ * listener and the retry-service produce identical classifications and a
+ * single behaviour change requires a single edit.
+ *
+ * Resolution order:
+ * 1. If the Throwable is a FormBindingApplicatorException, return its reasonCode().
+ *    Subclass dispatch handles SchemaConfig / Infra / DataIntegrity / Timeout
+ *    (Timeout extends Infra so it inherits 'temporary_error').
+ * 2. Otherwise, return 'unknown_error' — anything outside the hierarchy
+ *    (database connection lost not surfaced as Infra, generic RuntimeException
+ *    from a non-applicator code path, IdentityMatchInvariantViolation if it
+ *    somehow leaks here) is unknown from the response-shape perspective.
+ *
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ */
+final class FormBindingExceptionClassifier
+{
+    public static function classify(Throwable $exception): string
+    {
+        if ($exception instanceof FormBindingApplicatorException) {
+            return $exception->reasonCode();
+        }
+
+        return 'unknown_error';
+    }
+}