* fix: prevent Anthropic token fallback leaking to third-party anthropic_messages providers
When provider is minimax/alibaba/etc and MINIMAX_API_KEY is not set,
the code fell back to resolve_anthropic_token() sending Anthropic OAuth
credentials to third-party endpoints, causing 401 errors.
Now only provider=="anthropic" triggers the fallback. Generalizes the
Alibaba-specific guard from #1739 to all non-Anthropic providers.
* fix: set provider='anthropic' in credential refresh tests
Follow-up for cherry-picked PR #2383 — existing tests didn't set
agent.provider, which the new guard requires to allow Anthropic
token refresh.
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Co-authored-by: 0xbyt4 <35742124+0xbyt4@users.noreply.github.com>