GitLab

- Fix flaky WebSocket passthrough test: allow StatusNormalClosure after
  client close instead of requiring NoError (race condition fix)
- Fix ratelimit 401 test: use PlatformOpenAI instead of PlatformGemini
  for OAuth token cache invalidation scenario (more accurate)
- Add usageLoadQueue: Anthropic OAuth/setup-token accounts sharing the
  same proxy exit are serialized with 1-2s jitter to prevent upstream 429
- AccountUsageCell: add module-level usage cache (5min TTL), unmounted
  safety guard, and integrate enqueueUsageRequest for throttled fetching