fix: decode feature.json as UTF-8 in Windows PowerShell

[hamedrabah]

Description

Fixes #4333.

Windows PowerShell 5.1 decodes Get-Content -Raw with the active ANSI code page, so a BOM-less UTF-8 feature.json can corrupt non-ASCII feature paths before JSON parsing. Read both existing feature.json paths with System.IO.File.ReadAllText(..., Encoding.UTF8) instead.

The regression writes a BOM-less UTF-8 feature directory containing Chinese characters, invokes Get-FeaturePathsEnv under Windows PowerShell 5.1, and verifies the exact resolved path.

Testing

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

Additional checks:

• python3 -m py_compile tests/test_check_prerequisites_paths_only.py
• git diff --check
• The Windows-only regression is skipped on macOS. uv sync could not complete because the configured package registry was unreachable, so CI still needs to execute the test on Windows PowerShell 5.1.

AI Disclosure

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

Codex (GPT-5, autonomous) assisted with issue investigation, implementation, test design, and review. The resulting diff and repository requirements were manually reviewed before submission.

[Copilot]

Pull request overview

Fixes UTF-8 decoding of feature.json in Windows PowerShell 5.1.

Changes:

• Reads existing feature configuration explicitly as UTF-8.
• Adds a Windows PowerShell regression test for Unicode paths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
scripts/powershell/common.ps1	Uses UTF-8 decoding for both feature JSON reads.
tests/test_check_prerequisites_paths_only.py	Adds the Unicode-path regression test.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.