fix(gateway): add encoding=utf-8, errors=replace to subprocess calls on Windows

[ethanhuang996-alt]

Problem

On Windows Chinese locale (GBK/CP936), subprocess.Popen/run(text=True) defaults to GBK encoding. When a subprocess outputs UTF-8 Chinese characters, the internal _readerthread crashes.

This causes: output pipe stalls, event loop blocked, WebSocket write timeout, window freezes, gateway restarts. errors.log showed 1,830 identical crash entries.

Fix

Add encoding=utf-8, errors=replace to 5 subprocess calls in:

• tui_gateway/server.py (4 locations: _SlashWorker, quick command runner, API proxy)
• tui_gateway/git_probe.py (1 location: git probe)

Verification

• ast.parse() passes on both files
• 20 consecutive subprocess runs with UTF-8 Chinese output, zero crashes
• Environment: Windows 10 zh-CN, Python 3.12/3.14-rc

[teknium1]

Closing as superseded by the consolidated Windows console-flash work tracked in #54220.

The relevant pieces from this PR/cluster have now landed through the targeted follow-up PRs #54236, #53892, and #54417, or are recorded in the umbrella tracker for any remaining native-Windows verification. Keeping this separate PR open would duplicate the tracker and the merged follow-up work.

Thanks for digging into this — the reports and PRs in this cluster helped identify the remaining spawn legs.