feat(form-builder): FormFieldValidationRuleService + legacy backfill + snapshot + library row-copy

api/app/Exceptions/FormBuilder/UnknownValidationRuleTypeException.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+use RuntimeException;
+
+/**
+ * Raised by `FormFieldValidationRuleService::replaceRules()` (and the
+ * FormRequest validator in WS-5b commit 3) when a caller supplies a
+ * rule_type string that does not resolve to a `FormFieldValidationRuleType`
+ * enum case, or a parameter shape that mismatches the rule's schema.
+ */
+final class UnknownValidationRuleTypeException extends RuntimeException
+{
+}