fix: keep API errors out of terminal UI

[floze-the-genius]

Summary

Stops non-2xx Anthropic responses from being written directly through console.warn, where the raw line can overwrite OpenCode's terminal UI.

Structured fetch_error_response diagnostic logging remains unchanged. The response is still returned to OpenCode with its original status, headers, and body, so normal quota handling and retry UI continue to work.

Related issue

Closes #243.

Testing

• Added a deterministic mocked 429 regression with no live Anthropic request.
• Verified no terminal warning is emitted.
• Verified status 429, retry-after: 11218, and the JSON error body are preserved unchanged.
• Verified the outbound request occurs once.
• make all
  • lint and format clean
  • TypeScript build passes
  • 204 / 204 tests pass
• git diff --check

Checklist

• PR title follows Conventional Commits
• make all passes locally
• Tests added or updated where applicable
• README or docs updated where applicable (not needed; no user-facing configuration changed)