Follow-up: keep direct tabular queue failures from aborting streams

[Paul Lizer (paullizer)]

Summary:

• Removes local traceback imports in route_backend_chats.py that shadowed the module-level traceback import and masked streaming exceptions.
• Makes direct source-backed tabular queueing non-fatal: descriptor/credential failures now log a tagged warning and fall back to the existing tabular analysis path instead of aborting the stream.
• Adds regression coverage for non-fatal direct queue fallback.

Validation:

• .\.venv\Scripts\python.exe -m py_compile application\single_app\route_backend_chats.py application\single_app\config.py functional_tests\test_tabular_row_orchestration_scale.py
• .\.venv\Scripts\python.exe functional_tests\test_tabular_row_orchestration_scale.py