SessionDB WAL handles leak when streaming.py creates a new SessionDB
instance per request and replaces the cached agent's _session_db without
closing the old one. Each orphaned connection holds 2 FDs (.db +
.db-wal), causing FD exhaustion and EMFILE crashes after ~73 messages.
Fix: close the previous _session_db before replacing it on cached
agents, mirroring the close-before-replace pattern used elsewhere in the
codebase.
Wali Reheman
9b987eefb0