fix(sessions): surface gateway SSE failures and add polling fallback (#828)

* fix(sessions): surface gateway SSE failures and add polling fallback

- add a JSON probe mode for the gateway SSE endpoint
- detect watcher-unavailable 503s from the browser
- fall back to periodic session refresh with a toast
- add probe payload tests and endpoint coverage

Fixes #635

* fix(sessions): surface gateway SSE failures and add polling fallback (#826)

Absorbed from PR #826 by @cloudyun888 (fixes #635).

When the gateway watcher thread is not running, the browser now shows a
toast notification and falls back to 30-second periodic polling for session
sync. Previously the SSE failure was completely silent with no user feedback.

Changes from original PR:
- Deleted misplaced test_gateway_sse_probe_unit.py (was at repo root, not
  discovered by `pytest tests/`); unit tests moved into tests/test_gateway_sync.py
- _gateway_sse_probe_payload now checks watcher._thread.is_alive() rather
  than just watcher is not None — a watcher instance with a dead poll thread
  now correctly reports unavailable and activates the polling fallback
- probeGatewaySSEStatus catch(e) now starts the polling fallback on network
  error rather than silently swallowing the failure
- Added 5 unit tests covering all watcher-alive/dead/missing/disabled branches

Co-authored-by: cloudyun888 <269269188+86cloudyun-afk@users.noreply.github.com>

* cleanup(gateway): public is_alive() + dedup probe/live watcher-alive check + changelog

Three small cleanups on top of @cloudyun888's PR #826 absorption:

1. Add GatewayWatcher.is_alive() public accessor so routes.py doesn't
   reach into the private _thread attribute.  The existing private-
   attribute check stays as a defensive fallback for any older in-
   memory instance or test double that doesn't implement the full API.

2. Dedupe the watcher_alive computation in _handle_gateway_sse_stream:
   the live-SSE path now calls _gateway_sse_probe_payload(...) and reads
   its watcher_running field instead of re-deriving the same logic
   inline.  Keeps probe and SSE in sync automatically.

3. CHANGELOG trailer was (#826, fixes #635, @cloudyun888) — this PR is
   #828, so updated to (#828, absorbs PR #826 by @cloudyun888, fixes
   #635) matching the repo convention for absorbed PRs (see #805).

Added two regression tests:
- test_gateway_watcher_is_alive_public_method — covers the three
  lifecycle states (before start, while running, after stop).
- test_probe_payload_prefers_public_is_alive — asserts the probe
  uses watcher.is_alive() rather than poking _thread when the
  public method exists.

Full suite: 1735 passed, 0 new failures.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: cloudyun888 <269269188+86cloudyun-afk@users.noreply.github.com>
Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>
Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,18 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.140] — 2026-04-22
+
+### Fixed
+- **Gateway SSE sync failures now surface to the user** — when the gateway watcher
+  thread is not running, the browser now shows a toast notification and automatically
+  falls back to 30-second polling for session sync. Previously this failed silently
+  with no feedback. (#828, absorbs PR #826 by @cloudyun888, fixes #635)
+- `_gateway_sse_probe_payload` now checks `watcher._thread.is_alive()` rather than
+  just `watcher is not None`, so a watcher instance with a dead poll thread correctly
+  reports unavailable and triggers the polling fallback.
+- Probe fetch network errors now also activate the polling fallback as a safe default
+  rather than silently swallowing the failure.
+
 ## [v0.50.139] — 2026-04-22
 
 ### Fixed