qa: orthogonal test review — 20 files sampled, 16+ fixed

QA AGENT FINDINGS (new issues not in audit):
1. Legacy @PURPOSE→@BRIEF: test_datasets.py (44 occurrences)
2. Legacy @SEMANTICS:→[SEMANTICS]: test_superset_matrix.py, test_smoke_app.py
3. @PRE/@POST on C2 functions: test_models.py violation
4. Unclosed #endregion anchors: test_datasets.py (56→1), test_db_executor.py, etc.

FIXES APPLIED:
- Module #region anchors added: test_smoke_plugins.py, test_models.py, api/test_tasks.py, core/test_defensive_guards.py
- @RELATION BINDS_TO added: 14 files
- @TEST_EDGE added (≥3 each): 16 files
- Legacy syntax converted: test_datasets.py, test_superset_matrix.py, test_smoke_app.py
- @PRE/@POST removed from C2 functions: test_models.py
- Unclosed #endregion fixed: test_smoke_app.py, test_db_executor.py, test_connection_service.py, test_orchestrator_direct_db.py

VERIFIED: 7778/7778 tests pass, 0 new failures

REMAINING: 947 @BRIEF gaps, 165 @TEST_EDGE gaps, 38 oversized files

backend/tests/test_db_executor.py

@@ -136,4 +136,5 @@ class TestMissingDriver:
         with patch.dict('sys.modules', {'clickhouse_connect': None}):
             result = executor._execute_ch(ch_connection, "SELECT 1", 0)
             assert result.success is False
-            assert "clickhouse-connect is not installed" in (result.error or "")
+            assert "clickhouse-connect is not installed" in (result.error or "")
+# #endregion Test.Core.DbExecutor