fix(qa): address qa-tester findings — mock target_column, FetchModelsRequest schema, error handling

- Fix MagicMock poisoning: add target_column=None to _make_mock_job/mock_job fixtures (3 files)
- Add FetchModelsRequest Pydantic model for type-safe fetch-models endpoint
- logger.warning→logger.error for fetch_models 502 path
- Early validation: require api_key or provider_id in fetch-models
- 50 tests pass, 0 failures (test_clickhouse_insert_integration, test_orchestrator, test_preview)

backend/src/plugins/translate/__tests__/test_clickhouse_insert_integration.py

@@ -35,6 +35,7 @@ def _make_mock_job(**overrides):
     job.target_database_id = "1"
     job.environment_id = "test-env"
     job.context_columns = []
+    job.target_column = None
     for k, v in overrides.items():
         setattr(job, k, v)
     return job

backend/src/plugins/translate/__tests__/test_orchestrator.py

@@ -48,6 +48,7 @@ def mock_job() -> MagicMock:
     job.provider_id = "provider-1"
     job.batch_size = 50
     job.upsert_strategy = "MERGE"
+    job.target_column = None
     return job
 
 

backend/src/plugins/translate/__tests__/test_preview.py

@@ -35,6 +35,7 @@ def _make_mock_job(**overrides):
     job.batch_size = overrides.get("batch_size", 50)
     job.upsert_strategy = overrides.get("upsert_strategy", "MERGE")
     job.status = overrides.get("status", "DRAFT")
+    job.target_column = None
     return job
 # endregion _make_mock_job