fix(agent): clear _session_messages in AIAgent.close() (#42123)

close() is the hard teardown for true session boundaries (/new, /reset,
session expiry).  It already closes the OpenAI client and child agents but
left the conversation-history list intact.  Mirror the soft-eviction path
(_release_evicted_agent_soft clears _session_messages) so a held reference
to a closed agent — e.g. a draining background task — doesn't pin tens of
MB of tool outputs until the agent object itself is collected.

run_agent.py

@@ -3087,6 +3087,17 @@ class AIAgent:
         except Exception:
             pass
 
+        # 6. Free conversation history.  Mirrors _release_evicted_agent_soft's
+        # soft-eviction clear — close() is the hard teardown for true session
+        # boundaries (/new, /reset, session expiry), so the message list won't
+        # be reused.  Drops the reference proactively rather than waiting for
+        # the agent object itself to be collected, which matters when a caller
+        # still holds the closed agent (e.g. a draining background task).
+        try:
+            self._session_messages = []
+        except Exception:
+            pass
+
     def _hydrate_todo_store(self, history: List[Dict[str, Any]]) -> None:
         """
         Recover todo state from conversation history.