fix(test): add allow_messages_dispatch to group API contract test

The recent upstream commit added allow_messages_dispatch to the Group
DTO but did not update the API contract test expectation.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/server/api_contract_test.go

@@ -212,6 +212,7 @@ func TestAPIContracts(t *testing.T) {
 							"claude_code_only": false,
 						"fallback_group_id": null,
 						"fallback_group_id_on_invalid_request": null,
+						"allow_messages_dispatch": false,
 						"created_at": "2025-01-02T03:04:05Z",
 						"updated_at": "2025-01-02T03:04:05Z"
 					}