shouldMarkCreditsExhausted was blocked by isURLLevelRateLimit check when
credit overages retry returned "Resource has been exhausted (e.g. check quota).",
causing credits to never be marked as exhausted. This led to an infinite loop
where each request injected credits, bypassed model rate limits, and failed again.
- Remove isURLLevelRateLimit guard from shouldMarkCreditsExhausted (only called
for credit retry responses — if credits retry fails, mark exhausted)
- Add "resource has been exhausted" to creditsExhaustedKeywords
- Update tests to match corrected behavior
21b6f2d593