chore: update backend tests

backend/tests/test_db_executor.py

@@ -1,5 +1,9 @@
 # #region Test.Core.DbExecutor [C:3] [TYPE Module] [SEMANTICS test, database, executor, direct]
 # @BRIEF Pytest tests for DbExecutor — direct database INSERT execution.
+# @RELATION BINDS_TO -> [DbExecutor]
+# @TEST_EDGE: missing_connection -> Returns error "not found"
+# @TEST_EDGE: invalid_type -> Unsupported dialect returns error
+# @TEST_EDGE: external_fail -> Execution error handled gracefully
 # @TEST_CONTRACT DbExecutor.execute_sql routes to correct dialect driver and returns DbExecutionResult.
 # @TEST_CONTRACT Connection pool is reused across calls (same connection_id → same pool).
 # AUDIT_NOTE: patch.object targets _execute_pg/_execute_ch/_execute_mysql which are