Add common PowerShell functions for Specify

[Bee92-exe]

Description

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 (fill in the disclosure below)

AI disclosure: N/A

[Copilot]

Copilot review overview

🟡 Changes recommended

The fix is in an unreferenced duplicate file, while runtime consumers and the test still use the unchanged common.ps1.

Review effort: Balanced
Findings: 1

Open (1)

• Fix change in common.ps1 instead of unused common-fi.ps1 · New

What changed in this PR

Adds PowerShell helper logic intended to reject unusable python3 executables, plus regression coverage.

Changes:

• Adds a duplicate PowerShell common-functions file.
• Adds a cross-platform interpreter-selection regression test.

File	Description
scripts/​powershell/​common-fi.ps1	Adds duplicated common helpers with updated Python detection.
tests/​test_common_ps1_python3_command.py	Tests fallback from an unusable python3.

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[mnriem]

Thanks for taking the time to investigate this and add regression coverage. The underlying problem is valid: PowerShell should not select a python3 executable merely because Get-Command finds it, since the Windows Store alias can exist but fail when invoked.

I’m closing this PR because #4151 already addresses the same issue with broader coverage, including fallback to python and py -3, nonzero exit handling, and PowerShell error-preference behavior. This implementation also adds the change in common-fi.ps1, while runtime consumers and the test continue to load common.ps1, so the proposed fix is not active and its regression test currently fails.

Thank you again for the contribution. Consolidating the work in the existing PR will help us avoid competing implementations and keep the review focused.