fix: improve Windows Git Bash detection

[yuefengw]

Closes #38

This improves Git Bash detection on Windows by checking bash.exe directly, common Git for Windows install paths, git --exec-path, and git.exe-derived paths.

It also adds regression coverage for detecting Git Bash from PATH, git exec path, and git.exe candidates.

[qorzj]

感谢贡献。在DeepSeek的建议下附加了一个改动：在 findAllWindowsExecutableCandidates("bash") 结果中，过滤掉 System32\bash.exe