fix: differentiate server overload cooldown, add error classifier rules

[geminiyubing-jpg]

Summary

• credential_pool: Differentiate server overload cooldown from rate-limit cooldown for more precise backoff handling
• error_classifier: Add new error classification patterns to improve failover decision-making
• tests: Add corresponding test cases for error classifier

Test plan

• Run pytest tests/agent/test_error_classifier.py
• Verify credential pool cooldown logic handles server overload vs rate-limit correctly

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[alt-glitch]

Multiple open PRs fix the same issue (#14038): this PR, #14788, and #14055. Maintainers should pick one.

[teknium1]

Automated hermes-sweeper review: this PR's core fix is already implemented on current main.

Evidence:

• agent/error_classifier.py:136 now defines shared overload patterns including temporarily overloaded / service may be temporarily overloaded, with comments tying the behavior to "overloaded" server errors classified as rate_limit, exhausting credential pool #14038/Error classifier: 429 'temporarily overloaded' misclassified as rate_limit — triggers wrong recovery path #15297.
• agent/error_classifier.py:898 checks those overload patterns inside the HTTP 429 path before falling through to rate_limit; overload returns FailoverReason.overloaded without should_rotate_credential=True, while normal 429s still rotate at agent/error_classifier.py:912.
• tests/agent/test_error_classifier.py:389 covers the regression: a 429 body saying The service may be temporarily overloaded... classifies as overloaded with no credential rotation, and a genuine 429 rate limit still rotates.
• The implementing commit is 38e7bd8 (fix(agent): classify 429 'overloaded' bodies as overloaded, not rate_limit), which landed on origin/main and closed the shared issue "overloaded" server errors classified as rate_limit, exhausting credential pool #14038.

The prior maintainer note that several PRs overlapped on #14038 was accurate; main has since taken a later fix for that behavior.