fix(routing): asyncio.gather without return_exceptions=True leaks coroutines on partial document fetch failure #251

Summary

route_query() in routing.py uses asyncio.gather(*fetch_coros) without return_exceptions=True. If any single document fetch raises DependencyUnavailable, asyncio.gather immediately propagates the exception and cancels all remaining in-flight fetch coroutines, leaving them un-awaited (resource leak) and giving the caller no way to distinguish a single-document transient error from a total outage.

Root Cause

backend/app/api/routing.py around line 95:

fetch_coros = [_fetch_doc(doc_id, tenant_id, store) for doc_id in document_ids]
docs = await asyncio..gather(*fetch_coros)   # ← no return_exceptions=True

When _fetch_doc raises for doc_id[0], the gather cancels doc_id[1..N]'s coroutines mid-flight, and those coroutines' async DB sessions may not be properly released.

Failure Scenario

1. Caller submits document_ids=['doc_aaa', 'doc_bbb', 'doc_ccc'].
2. DB connection drops intermittently; _fetch_doc('doc_aaa') raises DependencyUnavailable.
3. asyncio.gather immediately propagates the exception; _fetch_doc('doc_bbb') and _fetch_doc('doc_ccc') are cancelled mid-flight.
4. Caller receives 503. But doc_bbb and doc_ccc are perfectly accessible.
5. Two SQLAlchemy async sessions are abandoned without close() → connection pool exhaustion under sustained traffic.

Proposed Fix

results = await asyncio.gather(*fetch_coros, return_exceptions=True)

# Separate exceptions from results:
docs = []
for doc_id, result in zip(document_ids, results):
    if isinstance(result, DependencyUnavailable):
        raise result          # re-raise infrastructure error
    elif isinstance(result, Exception):
        raise result          # unexpected error
    elif result is None:
        raise document_not_found(doc_id)   # now we know WHICH doc failed
    else:
        docs.append(result)

This guarantees all coroutines run to completion (or explicit error), sessions are properly released, and the 404 message correctly names the missing doc_id.