feat(form-builder): standardised error envelope for public form API (D6)

S2c D6. Seven concrete exceptions over a shared PublicFormApiException
base + a single renderer in bootstrap/app.php produce the contract:

  { "message": "...", "code": "...", "errors"?: {...} }

Codes: SCHEMA_NOT_FOUND (404), TOKEN_EXPIRED (410), TOKEN_REVOKED (410),
SCHEMA_UNPUBLISHED (410), SUBMISSION_ALREADY_SUBMITTED (409),
RATE_LIMITED (429 with Retry-After header), VALIDATION_FAILED (422
with per-field errors).

Used by PublicFormController (resolve) and PublicFormSubmissionController
(load/submit lifecycle). Every public-form endpoint now emits the same
envelope regardless of which branch failed; the renderer only fires on
PublicFormApiException so the authenticated API still uses its default
Laravel shapes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/Exceptions/FormBuilder/PublicFormApiException.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+use RuntimeException;
+
+/**
+ * Base for every public form-builder API error. Renders as the S2c D6
+ * standardised envelope:
+ *
+ *   { "message": "...", "code": "...", "errors"?: {...} }
+ *
+ * Concrete subclasses are tiny — they only set the machine code, HTTP
+ * status, and the user-facing message.
+ */
+abstract class PublicFormApiException extends RuntimeException
+{
+    /** @var array<string, array<int, string>>|null */
+    public ?array $fieldErrors = null;
+
+    /** @var array<string, string> */
+    public array $headers = [];
+
+    public function __construct(
+        public readonly string $publicCode,
+        public readonly int $status,
+        string $message,
+    ) {
+        parent::__construct($message);
+    }
+}