Wire AIAgent.close() into every gateway code path where an agent's
session is actually ending:
- stop(): close all running agents after interrupt + memory shutdown,
then call cleanup_all_environments() and cleanup_all_browsers() as
a global catch-all
- _session_expiry_watcher(): close agents when sessions expire after
the 5-minute idle timeout
- _handle_reset_command(): close the old agent before evicting it from
cache on /new or /reset
Note: _evict_cached_agent() intentionally does NOT call close() because
it is also used for non-destructive cache refreshes (model switch,
branch, fallback) where tool resources should persist.
Ref: #7131
fbe28352e4