fix(routing): 404 on batch document lookup doesn't identify which doc_id failed, forcing binary search on callers #277

Summary

When one of N document IDs in a route request is not found, route_query() raises document_not_found() with a generic 404 message that does not include the failing doc_id. With up to 25 document IDs in a batch request, the caller cannot identify which one is missing without performing a binary search.

Root Cause

backend/app/api/routing.py around line 96:

docs = await asyncio.gather(*[_fetch_doc(doc_id, ...) for doc_id in document_ids])

for doc in docs:
    if doc is None:
        raise document_not_found()   # ← which doc_id? No information provided

The doc_id corresponding to the None result is lost because the index into docs is not tracked alongside the fetch result.

Failure Scenario

1. Caller submits 25 document_ids to batch-route a query.
2. One of them — say document_ids[17] — was deleted 3 days ago.
3. The response is 404 {"code": "DOCUMENT_NOT_FOUND", "message": "No document with the given id exists."}.
4. Caller cannot tell which of 25 IDs failed.
5. Caller must make 25 individual /documents/{id} requests to binary-search for the missing one.

Proposed Fix

Track doc_id alongside the fetch result:

async def _fetch_doc_tagged(doc_id: str, ...) -> tuple[str, DocumentRecord | None]:
    return doc_id, await _fetch_doc(doc_id, ...)

tagged_results = await asyncio.gather(
    *[_fetch_doc_tagged(doc_id, ...) for doc_id in document_ids],
    return_exceptions=True,
)

for doc_id, doc in tagged_results:
    if doc is None:
        raise HTTPException(
            status_code=404,
            detail={
                "code": "DOCUMENT_NOT_FOUND",
                "document_id": doc_id,    # ← now caller knows
                "message": f"Document '{doc_id}' not found.",
            }
        )