fix(git): handle non-UTF-8 bytes in git_show

[dev-willbird1936]

Description

Make the Git server's git_show robust to valid commits whose patch contains bytes that are not valid UTF-8. Decode replacement bytes instead of failing the complete result with UnicodeDecodeError.

Root cause

The Git server strictly decoded GitPython patch bytes as UTF-8. One invalid byte could discard the complete patch result, including otherwise useful commit metadata.

Testing

• Focused git_show_handles_non_utf8_diff assertion passed.
• uv run ruff check passed.
• uv run pyright passed.
• uv build passed.
• The full Python module produced 44 passed test bodies plus 38 existing Windows fixture-teardown errors from open GitPython handles; this is not presented as a green full-suite result.
• LLM-client integration was not exercised; tests call the Git server implementation directly.

This is an ordinary correctness/robustness fix, not a security report. Hosted POSIX CI is recommended to remove the Windows-only fixture teardown noise. Maintainer review remains pending.