feat(form-builder): exception hierarchy for binding-apply pipeline

Per RFC-WS-6 §Q3 v1.3 addition 2 (binding hierarchy) + §Q2 (invariant exception).

- Refactored FormBindingApplicatorException from concrete final to abstract
  base. Constructor (submissionId, message, previous?) preserves submissionId
  as a public readonly property so D2's outer-transaction handler can write
  it structurally to form_submission_action_failures.context JSON without
  regex-parsing the message. Replaced public-readonly reasonCode property
  with abstract reasonCode(): string method.
- Added 3 reason-coded subclasses:
  - FormBindingSchemaConfigException -> 'schema_config_error' (422)
  - FormBindingInfraException -> 'temporary_error' (503, NOT final because
    Timeout extends it)
  - FormBindingDataIntegrityException -> 'data_integrity_error' (422)
- Added FormBindingApplicatorTimeoutException extending FormBindingInfraException
  (timeout = temporary infra issue from user perspective; reasonCode inherited).
- Added IdentityMatchInvariantViolation as a sibling DomainException — NOT
  in the FormBindingApplicatorException hierarchy because it's thrown
  outside the binding-applicator pipeline.
- Migrated 3 existing throw sites in FormBindingApplicator::apply():
  - 'no_transaction' -> FormBindingInfraException (developer-error wants
    infra-triage workflow: GlitchTip alert + retry-after)
  - 'no_schema' -> FormBindingSchemaConfigException
  - 'unknown_purpose' -> FormBindingSchemaConfigException
- Updated FormBindingApplicatorIntegrationTest::test_no_transaction_guard_present
  to assert against the new throw shape (FormBindingInfraException + new
  message string) while preserving the test's intent (guard exists in source).

Wiring (deadline wrapper, classifier integration in listener catch +
retry-service recordFailure) lands in D2.

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

api/app/Exceptions/FormBuilder/FormBindingInfraException.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+/**
+ * Infrastructure issue during binding apply.
+ *
+ * Examples: database connection lost, lock-for-update wait exceeded,
+ * race condition surfaced as a constraint violation that retry would
+ * resolve, applicator invoked outside a DB transaction (developer-error
+ * surfacing as infra-triage workflow).
+ *
+ * Maps to HTTP 503, failure_response_code='temporary_error'.
+ * User-facing copy: "Temporary issue, please try again." with retry-after.
+ *
+ * NOT final — FormBindingApplicatorTimeoutException extends this class.
+ *
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ */
+class FormBindingInfraException extends FormBindingApplicatorException
+{
+    public function reasonCode(): string
+    {
+        return 'temporary_error';
+    }
+}