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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

Replace process-memory sync.Map + per-model runtime state with a single
"AICredits" key in model_rate_limits, making credits exhaustion fully
isomorphic with model-level rate limiting.

Scheduler: rate-limited accounts with overages enabled + credits available
are now scheduled instead of excluded.

Forwarding: when model is rate-limited + credits available, inject credits
proactively without waiting for a 429 round trip.

Storage: credits exhaustion stored as model_rate_limits["AICredits"] with
5h duration, reusing SetModelRateLimit/isRateLimitActiveForKey.

Frontend: show credits_active (yellow ⚡) when model rate-limited but
credits available, credits_exhausted (red) when AICredits key active.

Tests: add unit tests for shouldMarkCreditsExhausted, injectEnabledCreditTypes,
clearCreditsExhausted, and update existing overages tests.

feat: implement resolveCreditsOveragesModelKey function to stabilize model key resolution for credit overages