[OTel] Improve session lifecycle management #172

Session boundaries are currently decided by timing heuristics (3s grace, 30s idle, 2s debounce, 60s orphan TTL) and then patched up by _reopen_session, orphan-log replay, _reextract_with_logs, and _absorb_orphan_for_trace.

Meanwhile the standard session.id semconv attribute is referenced exactly zero times.
Direction: treat session.id / gen_ai.conversation.id / agentevals.session_name as the authoritative boundaries and demote timers to last-resort GC.

This one needs design.