The /api/chat/stream_status handler did a membership test against
_active_streams followed by an indexed read of the same key. Between
those two ops, a sibling stream's finally block (or a stop / cleanup
path) can pop the entry, turning the indexed read into a KeyError that
bubbles up as a 500. The race is the exact one _stream_set was already
written to avoid; the comment on the helper at the top of the module
spells out why a single .get() is the right pattern here too.
Collapse the two-step into a single .get() call so the lookup either
returns the live record or None, and report 'detached' / 404 based on
that single read. No behavior change on the happy path; the failure
mode under concurrent stream cleanup is now handled deterministically.
Closes#658.
2d7d7b2412