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/FormBuilder/Bindings/FormBindingApplicator.php

@@ -7,6 +7,8 @@ namespace App\FormBuilder\Bindings;
 use App\Enums\FormBuilder\BindingTargetType;
 use App\Enums\FormBuilder\FormFieldBindingMergeStrategy;
 use App\Exceptions\FormBuilder\FormBindingApplicatorException;
+use App\Exceptions\FormBuilder\FormBindingInfraException;
+use App\Exceptions\FormBuilder\FormBindingSchemaConfigException;
 use App\FormBuilder\Purposes\PurposeRegistry;
 use App\Models\FormBuilder\FormSubmission;
 use Illuminate\Database\Eloquent\Model;
@@ -46,27 +48,25 @@ class FormBindingApplicator
     public function apply(FormSubmission $submission, ?string $sectionId = null): BindingPassResult
     {
         if (DB::transactionLevel() < 1) {
-            throw new FormBindingApplicatorException(
-                'no_transaction',
-                (string) $submission->id,
-                'FormBindingApplicator must be invoked inside DB::transaction',
+            throw new FormBindingInfraException(
+                submissionId: (string) $submission->id,
+                message: 'FormBindingApplicator must be invoked inside DB::transaction',
             );
         }
 
         /** @var \App\Models\FormBuilder\FormSchema|null $schema */
         $schema = $submission->schema;
         if ($schema === null) {
-            throw new FormBindingApplicatorException(
-                'no_schema',
-                (string) $submission->id,
+            throw new FormBindingSchemaConfigException(
+                submissionId: (string) $submission->id,
+                message: "schema null for submission {$submission->id}",
             );
         }
         $purposeValue = $schema->purpose->value;
         if (! $this->purposeRegistry->has($purposeValue)) {
-            throw new FormBindingApplicatorException(
-                'unknown_purpose',
-                (string) $submission->id,
-                "purpose '{$purposeValue}' not registered",
+            throw new FormBindingSchemaConfigException(
+                submissionId: (string) $submission->id,
+                message: "purpose '{$purposeValue}' not registered",
             );
         }
 
@@ -168,6 +168,7 @@ class FormBindingApplicator
         // Per-strategy matrix. RFC §3 Q7.
         if ($newValue === null) {
             $behaviour = $strategy->nullWinnerBehaviour();
+
             return match ($behaviour) {
                 'write' => null,
                 'noop' => self::NO_OP,