fix(ingest): fall back to gen_ai.conversation.id for every vendor - #1139

🟢 Confidence 4/5 · likely safe to merge
One deterministic ordering rule in run_predicates, covered by new tests for both the fallback and the vendor-key precedence, with no warehouse or migration change.
_{quality 100/100 · no findings · tests covered · risk medium}

Ingest now appends gen_ai.conversation.id after each matched vendor's own session keys, so spans from the 11 vendors that never listed it group into a session instead of one trace:<id> per trace. Contained to classification, with matching Rust tests; safe to merge.

• run_predicates tries gen_ai.conversation.id last for every vendor, unless the list already ranks it
• unknown:openinference now ranks the conversation id ahead of OpenInference's session.id
• Module doc and the session-turns.ts comment restate the new precedence

_{062ce5a · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}