fix: add 402 billing error hint to gateway error handler (#5220) (#10057)

* fix: hermes gateway restart waits for service to come back up (#8260)

Previously, systemd_restart() sent SIGUSR1 to the gateway, printed
'restart requested', and returned immediately. The gateway still
needed to drain active agents, exit with code 75, wait for systemd's
RestartSec=30, and start the new process. The user saw 'success' but
the gateway was actually down for 30-60 seconds.

Now the SIGUSR1 path blocks with progress feedback:

Phase 1 — wait for old process to die:
  ⏳ User service draining active work...
  Polls os.kill(pid, 0) until ProcessLookupError (up to 90s)

Phase 2 — wait for new process to become active:
  ⏳ Waiting for hermes-gateway to restart...
  Polls systemctl is-active + verifies new PID (up to 60s)

Success:
  ✓ User service restarted (PID 12345)

Timeout:
  ⚠ User service did not become active within 60s.
    Check status: hermes gateway status
    Check logs: journalctl --user -u hermes-gateway --since '2 min ago'

The reload-or-restart fallback path (line 1189) already blocks because
systemctl reload-or-restart is synchronous.

Test plan:
- Updated test to verify wait-for-restart behavior
- All 118 gateway CLI tests pass

* fix: add 402 billing error hint to gateway error handler (#5220)

The gateway's exception handler for agent errors had specific hints for
HTTP 401, 429, 529, 400, 500 — but not 402 (Payment Required / quota
exhausted). Users hitting billing limits from custom proxy providers
got a generic error with no guidance.

Added: 'Your API balance or quota is exhausted. Check your provider
dashboard.'

The underlying billing classification (error_classifier.py) already
correctly handles 402 as FailoverReason.billing with credential
rotation and fallback. The original issue (#5220) where 402 killed
the entire gateway was from an older version — on current main, 402
is excluded from the is_client_error abort path (line 9460) and goes
through the proper retry/fallback/fail flow. Combined with PR #9875
(auto-recover from unexpected SIGTERM), even edge cases where the
gateway dies are now survivable.

gateway/run.py

@@ -4020,6 +4020,8 @@ class GatewayRunner:
             _hist_len = len(history) if 'history' in locals() else 0
             if status_code == 401:
                 status_hint = " Check your API key or run `claude /login` to refresh OAuth credentials."
+            elif status_code == 402:
+                status_hint = " Your API balance or quota is exhausted. Check your provider dashboard."
             elif status_code == 429:
                 # Check if this is a plan usage limit (resets on a schedule) vs a transient rate limit
                 _err_body = getattr(e, "response", None)