fix: persist active account state after selection

[Emilvooo]

Summary

• persist activeAccountIndex and roundRobinCursor whenever account selection changes them
• keep openai-accounts.json in sync during normal selection and exclusion-based failover
• add coverage for live round-robin persistence and persisted failover state

Why

• the plugin updated the active account in memory, but some selection paths did not write the new state to disk
• that left openai-accounts.json behind the real runtime state and could make restarts resume from an outdated active account
• keeping the persisted selection state live makes sticky, round-robin, and failover behavior easier to reason about

Testing

• npm run typecheck
• npm test
• npm run build