test(rag): direct coverage for knowledge_store_rag._run() (#442)

[mayoka0]

Closes #442.

What

Adds backend/tests/test_knowledge_store_rag.py — a focused, test-only module that exercises knowledge_store_rag._run() directly. Previously only the helpers (_ks_query_helpers, query_rewriting_ks_rag) were covered, leaving _run()'s guard branches, multi-KS fan-out, per-store error handling, and context/source assembly untested.

No production code is touched.

Base branch

The knowledge_store_rag.py processor and its helpers live on feat/ks-vector-indexing-label, not main, so this PR targets that branch (the existing tests/test_query_rewriting_ks_rag.py lives there too). Happy to retarget if you'd prefer it elsewhere.

Coverage (10 tests)

Mirrors the patch-query_one_ks pattern of the existing test_query_rewriting_ks_rag.py (real Assistant, AsyncMocked query_one_ks, real serialize_assistant / _extract_sources):

• Guard branches — missing assistant, empty RAG_collections, no user message, separator-only RAG_collections
• Success path — single-KS context + source assembly, raw_responses, assistant_data
• Query construction — last user message is used; top_k and owner forwarded verbatim
• Multi-KS fan-out — every configured KS is queried; contexts and sources aggregated
• Per-store error handling — a failing KS is logged/skipped without sinking a healthy one; all-failing yields empty context
• Assembly edge — an empty-text chunk is still surfaced as a source but omitted from the combined context

Note on the issue description

The issue mentions a [N]-numbered context. The current _run() joins chunk texts with \n\n (no [N] prefixes), so the tests assert the actual combined-context + aligned-sources behaviour rather than numbering. If [N] numbering is the intended direction, that's a small production change I'm happy to follow up with in a separate PR.

Testing

asyncio_mode = auto (per backend/pytest.ini), so:

cd backend && python3 -m pytest tests/test_knowledge_store_rag.py -v

All 10 pass locally.

— Contributed by Mayoka Labs