fix(gateway): fail closed when session.delete can't enumerate active sessions

If a concurrent RPC mutates _sessions while session.delete is iterating
it (e.g. a parallel session.create on the thread pool), the bare except
swallowed the RuntimeError and let the delete proceed against a row
that may still be live.  Snapshot via list(_sessions.values()) and
return an error when even that raises, instead of treating "couldn't
check" as "no active sessions."

tests/test_tui_gateway_server.py

@@ -2835,6 +2835,32 @@ def test_session_delete_refuses_active_session(monkeypatch):
     assert called == [], "delete_session must not be called for active sessions"
 
 
+def test_session_delete_fails_closed_when_active_snapshot_raises(monkeypatch):
+    """Concurrent ``_sessions`` mutation from another RPC thread can raise
+    ``RuntimeError: dictionary changed size during iteration``.  When the
+    handler can't enumerate active sessions safely it must refuse the
+    delete (fail closed) rather than fall through and allow it."""
+
+    class _DB:
+        def delete_session(self, *a, **kw):
+            raise AssertionError("delete must not run when active snapshot fails")
+
+    class _ExplodingDict:
+        def values(self):
+            raise RuntimeError("dictionary changed size during iteration")
+
+    monkeypatch.setattr(server, "_get_db", lambda: _DB())
+    monkeypatch.setattr(server, "_sessions", _ExplodingDict())
+
+    resp = server.handle_request(
+        {"id": "1", "method": "session.delete", "params": {"session_id": "x"}}
+    )
+
+    assert "error" in resp
+    assert resp["error"]["code"] == 5036
+    assert "enumerate active sessions" in resp["error"]["message"]
+
+
 def test_session_delete_returns_4007_when_missing(monkeypatch):
     class _DB:
         def delete_session(self, sid, sessions_dir=None):

tui_gateway/server.py

@@ -2112,11 +2112,16 @@ def _(rid, params: dict) -> dict:
         return _db_unavailable_error(rid, code=5036)
     # Block deletion of any session currently bound to a live TUI session
     # in this process.  The picker hides the active session anyway, but a
-    # racing caller could still target it.
+    # racing caller could still target it.  Snapshot via ``list(...)``
+    # because ``_sessions`` is mutated by concurrent RPCs on the thread
+    # pool — iterating the dict directly can raise ``RuntimeError:
+    # dictionary changed size during iteration``.  If even the snapshot
+    # raises, fail closed (refuse the delete) rather than fail open.
     try:
-        active = {s.get("session_key") for s in _sessions.values() if s.get("session_key")}
-    except Exception:
-        active = set()
+        snapshot = list(_sessions.values())
+    except Exception as e:
+        return _err(rid, 5036, f"could not enumerate active sessions: {e}")
+    active = {s.get("session_key") for s in snapshot if s.get("session_key")}
     if target in active:
         return _err(rid, 4023, "cannot delete an active session")
     sessions_dir = get_hermes_home() / "sessions"