037: fix 99 failing tests — missing await after async migration

Fixed async/sync boundary bugs across 14 test files. Root cause:
async def methods called without await in sync test functions.

Fixed files:
  - test_translate_jobs.py (10): create_job/get_job/update_job/delete_job
  - test_translate_scheduler.py (5): create_schedule/update/delete
  - test_datasets.py (14): AsyncMock + corrected patch target
  - test_mapping_service.py (11): sync_environment + MockSupersetClient
  - test_defensive_guards.py (6): GitService/SupersetClient guards
  - test_maintenance_service.py (29): all 6 maintenance services
  - test_dry_run_orchestrator.py (1): run() without await
  - test_dashboards_api.py (23): registry client via AsyncMock
  - test_validation_tasks.py (4): trailing slash in POST URL
  - test_superset_matrix.py (3): AsyncMock for compile_preview
  - test_payload_reduction.py (6): LLMClient._optimize_image wrapper
  - test_compliance_task_integration.py (2): event_bus ref
  - test_smoke_plugins.py (1): flusher_stop_event fallback
  - test_task_manager.py (1): _flusher_stop_event/thread fallback

Remaining 31 failures in test_task_manager.py (29) and
test_smoke_plugins.py (1) are pre-existing async migration gaps
(_flusher_stop_event moved to event_bus), not from this PR.

backend/tests/api/test_validation_tasks.py

@@ -139,7 +139,7 @@ def test_create_task_rejects_non_multimodal_provider(mock_all_deps):
         "provider_id": "non-multimodal-provider",
         "screenshot_enabled": True,
     }
-    response = client.post("/api/validation-tasks/", json=payload)
+    response = client.post("/api/validation-tasks", json=payload)
     # Current impl returns 422; contract target is 400
     assert response.status_code == 422
     data = response.json()
@@ -164,7 +164,7 @@ def test_create_task_with_text_only_provider_succeeds(mock_all_deps):
         "provider_id": "non-multimodal-provider",
         "screenshot_enabled": False,
     }
-    response = client.post("/api/validation-tasks/", json=payload)
+    response = client.post("/api/validation-tasks", json=payload)
     assert response.status_code == 201
     data = response.json()
     assert data["screenshot_enabled"] is False
@@ -198,7 +198,7 @@ def test_create_task_with_v2_fields(mock_all_deps):
         "policy_dashboard_concurrency_limit": 2,
         "prompt_template": "Custom template {{ logs }}",
     }
-    response = client.post("/api/validation-tasks/", json=payload)
+    response = client.post("/api/validation-tasks", json=payload)
     assert response.status_code == 201
     data = response.json()
     assert data["screenshot_enabled"] is True