fix: keep rate-limited sessions on the failover account

[Emilvooo]

Summary

• fix sessions repeatedly retrying a known rate-limited account and showing the same account-switch toast on every prompt
• rebind the session to the failover account after rate limits or auth failures so later prompts stay on the working account
• skip stale session bindings that point to unavailable accounts and persist rate-limit state so restarts do not forget limited accounts

Why

• the plugin could switch from account 1 to account 2 after a rate-limit response, but the stored session binding still pointed at account 1
• that caused the next prompt in the same session to start on the limited account again, which retriggered the same failover loop
• rate-limit state was also only kept in memory, so restarts could bring the same limited account back into rotation too early

Testing

• npm run typecheck
• npm test
• npm run build