fix(windows): configure stdio as utf-8

[lihailong00]

Summary

• configure Windows stdout/stderr for UTF-8 at CLI startup when the stream supports reconfigure
• safely leave non-standard or locked streams unchanged
• avoid force_terminal, so redirected output behavior is not altered

Reproduction and verification

A strict cp936 stream rendering the real welcome logo deterministically raised UnicodeEncodeError on current main. After the fix:

• uv run pytest tests/core/test_windows_stdio.py tests/core/test_startup_imports.py -q — 11 passed
• Ruff, Ty, and Pyright — passed

This supersedes closed #1497 while addressing its reviewed redirected-output and encoding=None problems.

Closes #1436

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.