Improve error handling and logging in evaluation judge

[dgageot]

• Propagate stream errors instead of silently ignoring non-EOF errors
• Return idiomatic (passed, reason, error) from parseJudgeResponse instead of a custom struct with a parsed bool
• Add structured logging for parse failures and successful responses
• Wrap JSON parse errors with fmt.Errorf for proper error chaining

Assisted-by: cagent

[docker-agent]

Review Summary

✅ Code looks good! No issues found in the changed code.

The PR successfully improves error handling:

• Stream errors are properly propagated instead of being silently ignored
• The function signature change to (passed, reason, error) is idiomatic Go
• Error wrapping with fmt.Errorf provides proper error chains
• Structured logging adds helpful observability for debugging

All test cases align with the implementation and error paths are correctly handled.