fix: return null from getShellFromEnv when environment variables are not set

[roomote]

Description

This PR attempts to address Issue #8578. The user reported that the getShellFromEnv() function was returning hardcoded fallback values instead of properly checking environment variables.

Changes

• Modified getShellFromEnv() to return null instead of hardcoded fallback values when COMSPEC (Windows) or SHELL (Unix) environment variables are not set
• This ensures the function accurately reflects whether the shell path was obtained from environment variables
• Updated test comments to clarify the expected behavior with the fix

Technical Details

The fix ensures that getShellFromEnv() properly returns null when environment variables are not set, allowing the main getShell() function's fallback chain to work as intended:

1. First checks VS Code config
2. Then checks userInfo()
3. Then checks environment variables (now returns null if not set)
4. Finally falls back to getSafeFallbackShell()

Testing

• All existing tests continue to pass
• The fallback logic is properly handled by getSafeFallbackShell()
• Tests have been updated with comments to document the new behavior

Fixes #8578

Feedback and guidance are welcome!

---

Important

getShellFromEnv() now returns null when environment variables are not set, ensuring proper fallback logic in shell detection.

• Behavior:
  • getShellFromEnv() in shell.ts now returns null if COMSPEC (Windows) or SHELL (Unix) environment variables are not set.
  • Ensures getShell() function's fallback chain works as intended, checking VS Code config, userInfo(), environment variables, and finally getSafeFallbackShell().
• Testing:
  • Updated test comments in shell.spec.ts to clarify expected behavior with the fix.
  • All existing tests continue to pass, ensuring fallback logic is properly handled by getSafeFallbackShell().

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[roomote]

I found some issues that need attention before merging.

[roomote]

P1: Potential regression for unknown Unix-like platforms

The current implementation will return null for platforms that are not win32, darwin, or linux (e.g., freebsd, openbsd, sunos, aix). This means these platforms won't check the SHELL environment variable at all.

Before this PR, these platforms would have gotten a hardcoded fallback (/bin/bash for Linux path). While returning null allows the fallback chain to work, it skips checking SHELL entirely for these platforms.

Consider this alternative that checks SHELL for all Unix-like platforms:

Suggested change

	return null
	// On Unix-like systems, SHELL is the environment variable
	return env.SHELL || null
	}

This way, any Unix-like platform (including FreeBSD, OpenBSD, etc.) will check SHELL, and only return null if it's not set, allowing the proper fallback chain to work.