fix(tui-gateway): reap leaked slash_worker sessions on disconnect + active_list liveness (re-scoped onto current main)

Salvaged from #35626 (banditburai) and re-scoped after maintainers landed the
parent-death watchdog (slash_worker.py) and PTY process-group teardown
(pty_bridge.py) directly on main. Those pieces are intentionally NOT included
here — this carries only what is still missing:

- C1 disconnect reap: ws.py's `finally` only re-pointed the dead transport at
  stdio. `_close_sessions_for_transport` now reaps `close_on_disconnect`
  sessions and schedules the grace-reap for the rest, offloaded via
  `asyncio.to_thread` so the blocking worker.close() + DB write never stalls
  the uvicorn loop.
- C2 create/close orphan race: `_attach_worker` stores the worker iff
  `_sessions.get(sid) is session` under the lock (else closes it), applied at
  every spawn site incl. the post-turn `_restart_slash_worker`.
- Single idempotent teardown funnel: session.close, WS disconnect, the
  generous-TTL idle reaper, shutdown, and the WS grace-reap all reach
  `_close_session_by_id` → `_teardown_session`; `_finalized`/`_closed` flags
  make concurrent/double teardown a no-op. `_sessions_lock` upgraded to RLock.
- uvicorn `ws_ping_interval/timeout=20s` so a half-open socket (reverse-proxy
  524) becomes a `WebSocketDisconnect` and the C1 path runs.

Plus two review-driven hardening fixes (mine):

- `session.active_list` now skips `_finalized` sessions so the footer
  "N sessions" count reflects attachable sessions instead of only ever
  growing until restart (#38950). Keys on `_finalized` only, NOT the stdio
  sentinel, so a standalone `hermes --tui` session stays visible.
- `_schedule_ws_orphan_reap._reap` pops via `_close_session_by_id`
  (under `_sessions_lock`) instead of `_sessions.pop` under the unrelated
  `_session_resume_lock` (#39591); the resume_lock now only guards the orphan
  re-check against `session.resume`.
- Float env knobs (`HERMES_SLASH_WATCHDOG_*`, `HERMES_TUI_SESSION_TTL_S`)
  parse with a fallback helper so a malformed value can't crash the worker at
  import.

Fixes #32377
Fixes #38950
Addresses #22855

Co-authored-by: banditburai <123342691+banditburai@users.noreply.github.com>
Co-authored-by: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>

web/src/components/ChatSidebar.tsx

@@ -120,7 +120,11 @@ export function ChatSidebar({ channel, className }: ChatSidebarProps) {
         if (cancelled) {
           return;
         }
-        return gw.request<{ session_id: string }>("session.create", {});
+        // close_on_disconnect: the gateway reaps this sidecar session (and its
+        // slash_worker subprocess) when the WS drops, instead of leaking it.
+        return gw.request<{ session_id: string }>("session.create", {
+          close_on_disconnect: true,
+        });
       })
       .then((created) => {
         if (cancelled || !created?.session_id) {