fix: address PR review comments

- Rename regionPromises → regionCache (BYK suggestion)
- Evict rejected promises from regionCache via .catch handler
  so retries after re-authentication work (Seer + BugBot)
- Use limit.map helper instead of manual map + Promise.all (BYK)
- Replace setTimeout + Promise.race with AbortSignal.timeout —
  self-cleaning, doesn't hold event loop (BYK + BugBot)
- Queued tasks check signal.aborted before work (code-scanner
  earlyExit pattern from PR #414)
- Take AGENTS.md from origin/main to resolve merge conflict

Author: BYK