feat: implement multi-account load balancing and 429 retry logic

[PSU3D0]

Summary

This PR introduces a multi-account load-balancing architecture for the Gemini OAuth plugin, allowing users to "stack" multiple Google accounts to maximize throughput and bypass single-account rate limits.

Changes

• Multi-Account Registry: Refactored the refresh token storage to support a semicolon-separated pool of account credentials.
• Load Balancing Fetch: Implemented a randomized account selection strategy in the plugin's fetch wrapper.
• Reactive Rate Limiting: Added a 429 error interceptor that tracks rate-limited accounts in memory using Retry-After headers, temporarily removing them from the pool to maintain high throughput.
• Cumulative Login Flow: Updated the OAuth callback logic to append new accounts to the pool instead of overwriting existing ones.
• Granular Token Management: Refactored token refresh logic to handle accounts individually, improving resilience against single-token revocations.

Impact

Users can now log in multiple times via opencode auth login google to aggregate the quotas of multiple GCP projects/accounts, effectively load balancing requests at the plugin level

[PSU3D0]

Not ready for prime time. Corner cases