fix: correct failing unit tests for product and import job services

- test_create_import_job_database_error: Monkeypatch db.flush instead
  of db.commit since service uses flush() per architecture rules
- test_create_product_already_exists: Store product_id before exception
  and rollback session to clear PendingRollbackError state
- test_delete_product_success: Add db.flush() to delete_product service
  method to ensure deletion is applied before verification

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/services/test_marketplace_service.py

@@ -59,10 +59,10 @@ class TestMarketplaceImportJobService:
             marketplace="Amazon",
         )
 
-        def mock_commit():
-            raise Exception("Database commit failed")
+        def mock_flush():
+            raise Exception("Database flush failed")
 
-        monkeypatch.setattr(db, "commit", mock_commit)
+        monkeypatch.setattr(db, "flush", mock_flush)
 
         with pytest.raises(ValidationException) as exc_info:
             self.service.create_import_job(db, request, test_vendor, test_user)