fix(tui): close AIAgent on session teardown to prevent FD leak

session.close only closed the slash_worker subprocess but never called
agent.close() on the AIAgent instance.  In the long-lived TUI gateway
process, this left httpx clients for GC to finalize.  When the OS
recycled a closed FD number for a new active connection, the stale
finalizer would close the live socket, causing intermittent
[Errno 9] Bad file descriptor on subsequent LLM API calls.

Call agent.close() (which properly shuts down the httpx transport pool
and TCP sockets) before closing the slash_worker.

tui_gateway/server.py

@@ -2439,6 +2439,12 @@ def _(rid, params: dict) -> dict:
         unregister_gateway_notify(session["session_key"])
     except Exception:
         pass
+    try:
+        agent = session.get("agent")
+        if agent and hasattr(agent, "close"):
+            agent.close()
+    except Exception:
+        pass
     try:
         worker = session.get("slash_worker")
         if worker: