fix(tests): don't run PowerShell tests via WSL-interop powershell.exe

[lissy93]

Description

Fixes tests for WSL.

Previously, tests were failing when running from within WSL. Because the script was checking if Powershell is within the path then running the PS1 fixtures if found, else skipping.

However, on WSL powershell.exe IS in the path, however Linux can't execute the powershell scripts.

This change just adds if os.name == "nt" to the powershel check statement, so has the same logic as the Linux tests, and sensibly skips windows tests when not on windows. Now everything passes :)

Fixes #2970

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

I did not use AI for any of the changes or debugging
But I did use it for a quick code review/ sanity check before submitting. Qwen 3 8B in ollama.
(And maybe Copilot will jump in here too with some more AI review 😉)

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates PowerShell detection in test fixtures to treat powershell(.exe) as Windows-only, avoiding non-Windows environments accidentally selecting Windows PowerShell.

Changes:

• Gate _POWERSHELL discovery behind os.name == "nt" in multiple tests.
• Normalize the detection expression with explicit parentheses for readability.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/test_setup_tasks.py	Restricts _POWERSHELL detection to Windows.
tests/test_setup_plan_no_overwrite.py	Restricts _POWERSHELL detection to Windows.
tests/test_setup_plan_feature_json.py	Restricts _POWERSHELL detection to Windows.
tests/test_check_prerequisites_paths_only.py	Restricts _POWERSHELL detection to Windows.

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[mnriem]

Windows PowerShell versus a user installed PowerShell is probably how we should differentiate and then make sure it works properly. Please address Copilot feedback in that light

[lissy93]

Thanks @mnriem - done :)