fix(routing): isinstance(result, Exception) misses asyncio.CancelledError -- appends to docs list causing AttributeError #284

Summary

asyncio.CancelledError is a BaseException (not Exception) in Python 3.8+. With return_exceptions=True in asyncio.gather, a cancelled _fetch_doc coroutine returns a CancelledError as the result value. The check isinstance(result, Exception) at routing.py:113 returns False for CancelledError. It is also not None, so docs.append(result) runs, appending the exception object to the docs list. Then sum(d.total_pages for d in docs) raises AttributeError: 'CancelledError' object has no attribute 'total_pages'.

Root cause

Fix #251 changed to asyncio.gather(..., return_exceptions=True) but used isinstance(result, Exception) which does not cover BaseException subclasses.

Fix

backend/app/api/routing.py line 113: change check to isinstance(result, BaseException) to also catch CancelledError, KeyboardInterrupt, and SystemExit.

Impact

Any client disconnect or outer task cancellation during multi-doc routing crashes with AttributeError instead of propagating a clean CancellationError.