* fix(agents): classify generic provider errors for failover
Anthropic returns bare 'An unknown error occurred' during API instability
and OpenRouter wraps upstream failures as 'Provider returned error'. Neither
message was recognized by the failover classifier, so the error surfaced
directly to users instead of triggering the configured fallback chain.
Add both patterns to the serverError classifier so they are classified as
transient server errors (timeout) and trigger model failover.
Closes#49706Closes#45834
* fix(agents): scope unknown-error failover by provider
* docs(changelog): note provider-scoped unknown-error failover
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Co-authored-by: Aaron Zhu <aaron@Aarons-MacBook-Air.local>
Co-authored-by: Altay <altay@uinaf.dev>
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