fix: reconfigure stdio to UTF-8 on Windows

[oussamalembarki]

Fixes #1242.

This PR adds a platform check to reconfigure sys.stdin and sys.stdout to use UTF-8 encoding when running on Windows. This prevents UnicodeDecodeError when the MCP server receives non-ASCII characters (emojis, CJK, etc.) from clients.

[igorls]

Thanks @oussamalembarki! Closing as duplicate — #1280 is a strict superset of this fix and is the canonical PR for the Windows UTF-8 stdio cluster. It covers everything here (mcp_server stdin/stdout reconfigure) plus:

• stderr coverage and errors="replace" so the JSON-RPC loop never crashes on undecodable bytes
• import-time _force_utf8_stdio() call that fixes a stdout-redirect race
• hooks_cli.run_hook reconfigure (this PR doesn't touch hooks)
• ensure_ascii=False in JSON-RPC responses so CJK round-trips without \uXXXX escapes
• a real cp936 subprocess regression test

Please follow #1280 (and #1282 for the CLI / fact-checker side, which neither this PR nor #1280 covers). Really appreciate the diagnosis — confirming the fix on multiple forks gives us high confidence it works.