Fix Windows compatibility in test_package.py script

[colloceo]

What was wrong?

The test_package.py script had hardcoded Unix-style paths (bin/pip) that would fail on Windows systems. Additionally, the activation command message only showed Unix instructions, making it confusing for Windows users. The code also had duplication of the pip path logic between create_venv() and install_wheel() functions, and used a generic Exception type instead of a more specific exception.

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Closes # (leave blank if none)

How was it fixed?

• Introduced a get_pip_exe() helper function to centralize platform-specific pip executable path detection (Windows: Scripts/pip.exe, Unix: bin/pip)
• Updated both create_venv() and install_wheel() to use the new helper function, eliminating code duplication
• Added platform-specific activation command instructions (Windows: Scripts\activate, Unix: source bin/activate)
• Changed generic Exception to ValueError for better error handling when multiple or no wheels are found

Todo:

• Clean up commit history
• Add or update documentation related to these changes (N/A - internal test script)
• Add entry to the release notes (N/A - internal test script change)

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