feat: findExecutable

[dojineko]

Summary

Fixed an issue where non-executable files could be passed to Spawn on Windows.

Problem

• In Windows environments, when using the where command to locate gemini, files without extensions may appear as candidates (example below)
• The current implementation does not specify shell: true for the child_process spawn function
  • https://nodejs.org/api/child_process.html#spawning-bat-and-cmd-files-on-windows
• This causes ENOENT: no such file or directory, uv_spawn 'gemini' error when files without extensions are passed

# Example
$ where gemini
C:\path\to\gemini
C:\path\to\gemini.cmd

Solution

• Changed command listing to not depend on external commands
• On Windows, search for the gemini executable while respecting the PATHEXT environment variable that indicates executable files
  • Avoided specifying shell: true for the spawn function as it caused integration tests to fail

Changes

• Added findExecutable() function to index.ts
  • Implementation equivalent to which or where commands
• Modified decideGeminiCliCommand() in index.ts to use findExecutable() when searching for executable files
  • Added related unit tests

Testing

• Tested on Windows 11: Confirmed that gemini-cli - chat (MCP)(prompt: "hello") executes successfully
• Tested on Ubuntu (WSL): Confirmed that gemini-cli - chat (MCP)(prompt: "hello") executes successfully
• Confirmed that prepared tests work on both platforms