Fallback discards retrieved evidence when raw_answer is empty and generation fails

[lucaosti]

Found by

test_stream_dispatch_final_fallback_uses_raw_answer_not_the_evidence_pack in #171 (services/ai/tests/unit/test_streaming_dispatch.py), while auditing test coverage for #154.

What happens

Both study_service.dispatch() and study_service.stream_dispatch() share the same final-fallback branch, used when generation fails (or, for streaming, when the stream yields nothing and the buffered retry also fails):

answer = raw_answer or "No relevant content found."

raw_answer is the bare answer string QVAC's own /query endpoint returns alongside the retrieved chunks — not pack.context_block(), the assembled, deduplicated evidence the rest of the pipeline treats as the actual retrieved content.

The gap

If raw_answer happens to be empty while pack.chunks holds real retrieved passages — which QVAC can return, e.g. when its own answer field is blank but sources are populated — the student sees "No relevant content found." even though relevant content was retrieved and is sitting right there in the pack.

This contradicts the graceful-degradation principle stated in docs/overview.md:

With generation disabled, every study action still returns source passages, so the system remains usable on memory-constrained machines.

Here generation isn't even disabled — it just failed — and the evidence still gets dropped.

Suggested fix

Prefer the pack when it has content:

answer = pack.context_block() or raw_answer or "No relevant content found."

This is already the pattern used one branch up, in the not meta.generation_required or rag_only path of dispatch() — so the fix is bringing the failure-fallback branch in line with the retrieval-only branch's existing behaviour, not inventing a new pattern.

Scope

Two call sites, both in services/ai/app/services/study_service.py:

• _route()'s else branch (non-streaming dispatch())
• stream_dispatch()'s final else branch

Tasks

• Change both branches to prefer pack.context_block()
• Update/replace test_stream_dispatch_final_fallback_uses_raw_answer_not_the_evidence_pack — it currently documents the old behaviour on purpose; once fixed it should assert the passage text is present
• Check for an equivalent test on the non-streaming dispatch() path and add one if missing
• Confirm this doesn't change behaviour when pack.chunks is genuinely empty (should still fall through to raw_answer, then the generic message)

Acceptance criteria

• A retrieval that returns real chunks but an empty raw_answer, combined with a generation failure, surfaces the retrieved passages rather than "No relevant content found."
• Behaviour is identical between dispatch() and stream_dispatch() for this branch, as it already is everywhere else in the file