Classify G1 credit exhaustion as quota exhaustion

[eddieparc]

Summary

Gemini/Antigravity failures with Google RPC reason INSUFFICIENT_G1_CREDITS_BALANCE should be classified as quota exhaustion so account rotation can move away from the exhausted account.

Observed error

429 RESOURCE_EXHAUSTED
Resource has been exhausted (e.g. check quota).
reason: INSUFFICIENT_G1_CREDITS_BALANCE

Why this matters

This is account credit exhaustion, not transient model capacity. If it is parsed as unknown/transient, the caller can retry the same exhausted account instead of marking it quota-limited and selecting another Antigravity account.

Patch

I have a small patch that maps the explicit reason string and matching message text to QUOTA_EXHAUSTED, with regression coverage in src/plugin/accounts.test.ts.

Validation:

npm run typecheck
npm run build
npm test

All pass locally.