Scoping the trace key by turn_id (the prior commit) fixed cross-turn
collisions but introduced a slow leak: _finish_trace only pops a key when a
turn ends cleanly (final response has content and no tool calls), so any
turn that is interrupted, ends on a tool call, or has empty final content
now leaves its uniquely-keyed entry in _TRACE_STATE forever. Previously the
constant per-session key was overwritten by the next turn, capping growth at
~1 entry per session.
Add an LRU cap (_MAX_TRACE_STATE) enforced by _evict_stale_locked, called
under _STATE_LOCK immediately before each insert. It evicts the
least-recently-updated entries (using the previously-dead last_updated_at
field) and ends their root span so nothing dangles. Regression test drives
50 non-finalizing turns against a cap of 8 and asserts the dict stays bounded
with the most-recent turns surviving.
f4fbaa6cda