## Summary
Reliability and authorization fixes for realtime chat sessions:
- Session-stream waitpoint delivery is scoped to the environment, so two
environments using the same session `externalId` can no longer complete
each other's waitpoints.
- The session snapshot-url routes now enforce per-session authorization,
and appending to a session's `out` channel requires secret-key auth, so
a session-scoped token can't read another session's snapshot or forge
assistant output.
- Appends that carry an `X-Part-Id` header are deduplicated on retry, so
a retried send can't duplicate a message.
- Session creation rejects expired sessions (instead of triggering a run
that can never receive input), `externalId` is immutable after creation,
and the sessions list endpoint returns friendly `run_*` ids to match the
single-session routes.
## Rollout
The waitpoint cache key gains an environment prefix. To keep waitpoints
registered by the previous deploy working across the boundary, the drain
reads both the new and the previous key for this release; the legacy
read can be removed a release later once no pre-deploy waitpoints
remain.
Eric Allam
b82d100483