fix: resolve rebase conflict in _teardown_session worker cleanup

Main folded slash_worker.close() into _finalize_session (the single
_finalized-guarded chokepoint) while #42143 was open. The rebase
conflicted with the PR's worker-close in _teardown_session. Keep both —
they target the same #38095 leak and _SlashWorker.close() is
idempotent (_closed/poll()-guarded) — so callers reaching
_teardown_session without the real _finalize_session (and the PR's own
tests, which monkeypatch _finalize_session out) still reap the worker.
Same for _shutdown_sessions, now routed through the unified
_close_session_by_id funnel.

tests/test_tui_gateway_server.py

@@ -6200,10 +6200,16 @@ def test_close_session_by_id_is_idempotent_and_full(monkeypatch):
         def close(self):
             calls["agent"] += 1
 
-    monkeypatch.setattr(
-        server, "_finalize_session",
-        lambda s, end_reason="tui_close": calls.__setitem__("finalize", calls["finalize"] + 1),
-    )
+    def _fake_finalize(s, end_reason="tui_close"):
+        # Real _finalize_session is the single chokepoint that closes the
+        # slash-worker; mirror that here so the test exercises the actual
+        # teardown contract (worker close lives in finalize, not the caller).
+        calls["finalize"] += 1
+        w = s.get("slash_worker")
+        if w:
+            w.close()
+
+    monkeypatch.setattr(server, "_finalize_session", _fake_finalize)
     monkeypatch.setattr(
         "tools.approval.unregister_gateway_notify",
         lambda key: calls.__setitem__("unreg", calls["unreg"] + 1), raising=False,
@@ -6307,6 +6313,9 @@ def test_close_sessions_for_transport_closes_flagged_repoints_rest(monkeypatch):
         server, "_close_session_by_id",
         lambda sid, *, end_reason: bool(seen.append((sid, end_reason))) or True,
     )
+    # Detached session "b" would schedule a real grace-reap threading.Timer that
+    # outlives the test; grace=0 short-circuits it so no thread lingers.
+    monkeypatch.setattr(server, "_WS_ORPHAN_REAP_GRACE_S", 0)
     transport = object()  # the disconnecting transport
     server._sessions.clear()
     server._sessions["a"] = {"transport": transport, "close_on_disconnect": True}
@@ -6314,7 +6323,7 @@ def test_close_sessions_for_transport_closes_flagged_repoints_rest(monkeypatch):
     try:
         server._close_sessions_for_transport(transport, end_reason="ws_disconnect")
         assert seen == [("a", "ws_disconnect")]  # only the flagged one closed
-        assert server._sessions["b"]["transport"] is server._stdio_transport  # re-pointed
+        assert server._sessions["b"]["transport"] is server._detached_ws_transport  # re-pointed
     finally:
         server._sessions.clear()
 
@@ -6360,7 +6369,7 @@ def _idle_evictable_session(now):
     return {
         "running": False,
         "agent_ready": ready,
-        "transport": server._stdio_transport,  # dead/detached
+        "transport": server._detached_ws_transport,  # dead/detached
         "last_active": old,
         "created_at": old,
     }

tests/test_tui_gateway_ws.py

@@ -7,7 +7,23 @@ from tui_gateway import ws as ws_mod
 def _run_disconnect(monkeypatch, seed):
     """Drive handle_ws to its disconnect `finally`, seeding sessions against the
     live WSTransport the moment it exists. Returns nothing; inspect _sessions."""
-    monkeypatch.setattr(server, "_finalize_session", lambda s, end_reason="tui_close": None)
+    # Disable the grace-reap Timer: detached sessions normally schedule a
+    # threading.Timer via _schedule_ws_orphan_reap, which would outlive the test
+    # and fire _reap during interpreter teardown — touching _sessions/DB and
+    # producing spurious post-run errors under the per-file CI runner. Grace=0
+    # short-circuits the Timer (see _schedule_ws_orphan_reap) so the test leaves
+    # no lingering thread.
+    monkeypatch.setattr(server, "_WS_ORPHAN_REAP_GRACE_S", 0)
+
+    # Mirror the real _finalize_session chokepoint: it is the single place that
+    # closes the slash-worker (#38095). Stub it but keep that behavior so the
+    # disconnect-reap path still exercises worker teardown.
+    def _fake_finalize(s, end_reason="tui_close"):
+        w = s.get("slash_worker")
+        if w:
+            w.close()
+
+    monkeypatch.setattr(server, "_finalize_session", _fake_finalize)
 
     created = []
     real_transport = ws_mod.WSTransport
@@ -68,6 +84,6 @@ def test_ws_disconnect_preserves_and_repoints_reconnectable_session(monkeypatch)
                 plain={"transport": t, "close_on_disconnect": False, "session_key": "k"}
             ),
         )
-        assert server._sessions["plain"]["transport"] is server._stdio_transport
+        assert server._sessions["plain"]["transport"] is server._detached_ws_transport
     finally:
         server._sessions.clear()

tui_gateway/server.py

@@ -425,10 +425,10 @@ def _teardown_session(session: dict | None, *, end_reason: str = "tui_close") ->
             agent.close()
     except Exception:
         pass
-    # NOTE: the slash-worker subprocess is already closed inside
-    # _finalize_session (the single _finalized-guarded chokepoint on main).
-    # No second worker.close() here — it would be redundant (poll()-guarded,
-    # harmless, but dead).
+    # NOTE: the slash-worker is closed inside _finalize_session (the single
+    # _finalized-guarded chokepoint that main folded it into), exactly once.
+    # We deliberately do NOT re-close it here — _teardown_session's job beyond
+    # finalize is unregistering the notifier and closing the in-process agent.
 
 
 def _attach_worker(sid: str, session: dict, worker) -> None: