* feat: add Slack reaction feedback to LangSmith
Record Slack reaction feedback against explicitly mapped LangGraph runs so user ratings are idempotent and tied to the message they reacted to.
* Address review feedback on Slack reaction → LangSmith feedback
- langsmith.py: drop lru_cache on _build_langsmith_feedback_clients so
rotated keys / late env hydration are picked up; dedupe by (key, url)
tuple instead of key alone so the same key pointing at different
endpoints (cloud + self-hosted) builds both clients.
- langsmith.py: treat LangSmithNotFoundError on delete_feedback as
success — out-of-order or redelivered reaction_removed events would
otherwise loop forever on Slack's retry policy.
- slack_feedback.py: include channel_id in _feedback_key so the same
message_ts in two channels can't collide on the same feedback id.
- slack_feedback.py: treat conflicting +/- reactions from one user as
ambiguous (clear feedback) instead of averaging to a misleading 0.5.
- slack_feedback.py + slack.py + webapp.py: gate reaction handling to
the user who triggered the run (stored in the slack_run_map mapping
alongside run_id). Prevents bystanders in shared channels from
polluting eval feedback.
