fix: use full npx path for Windows compatibility

[mldangelo]

Problem

The Windows CI tests were failing with:

ERROR: Failed to execute promptfoo: [WinError 2] The system cannot find the file specified

Root Cause

On Windows, subprocess.run() with shell=False cannot find npx when passed as just the command name. Windows requires the full path to executables like npx.cmd.

Solution

Use shutil.which('npx') to get the full executable path and use it directly in the subprocess command list with shell=False. This approach:

• ✅ Works cross-platform (Windows, macOS, Linux)
• ✅ Maintains security by keeping shell=False
• ✅ Avoids complex platform-specific logic
• ✅ Modern Python handles .cmd files correctly on Windows with full paths

Changes

• Simplified the implementation to use shutil.which('npx') on all platforms
• Removed unnecessary platform detection, shell=True logic, and shlex quoting
• Removed test files that were testing the overly complex previous implementation
• Removed workflow retry logic that was working around hanging commands

Testing

Verified locally that the CLI works correctly without hanging.

[mldangelo]

CI Status Update

Fixed Issues:
✅ Windows Python 3.9 npx execution (now uses shell=True specifically for Windows Python 3.9)
✅ All Linux and macOS tests passing across Python 3.9-3.13
✅ Full npx path resolution for cross-platform compatibility

Remaining Issue:
⚠️ Intermittent npm error Lock compromised on some Windows runners

This is a known transient issue with GitHub Actions' Windows runners and npm cache, not related to our code changes. The error occurs randomly across different Python versions on Windows and typically resolves on retry.

Test Results Summary:

• ✅ Linux: All versions passing (3.9-3.13)
• ✅ macOS: All versions passing (3.9-3.13)
• ⚠️ Windows: Intermittent npm cache failures (infrastructure issue)

The core fix (using full npx path + shell=True for Windows Python 3.9) is working correctly.