Upsert one test-case result with scores

Upsert one test-case result. Idempotent on (run_id, test_case_id). trace_id may be null now and set on a later call (out-of-order arrival).

scores is genuinely optional and carries three distinct meanings, so the out-of-order flow (POST the result with its scores, then re-POST later just to attach the OTel trace_id) cannot destroy them: absent → leave the existing scores untouched, [] → clear them, non-empty → replace them. Do not give this a default_factory=list, which would collapse the first two cases.

Every optional field is a partial update: a key the caller omits keeps its stored value, while an explicit null clears it. Sending scores replaces the result's scores ([] clears them); omitting it leaves them alone.