Force UTF-8 mode for Windows Python scripts

[aouxwoux]

For Windows launches, set PYTHONUTF8=1 on the process environment in the shared PythonScript launch path. This makes direct Python and pixi-launched plugin scripts use UTF-8 consistently, avoiding system-locale decoding of plugin output.

Adds a Windows regression test that launches a temporary Python script and verifies sys.flags.utf8_mode is enabled.

Validation:

• git diff --check
• Confirmed locally that PYTHONUTF8=1 enables Python UTF-8 mode
• Full CMake/Qt test suite was not run locally because this environment lacks CMake and Qt

Fixes #1618.

[ghutchis]

Hi - as part of our policy, we require commits to be signed by full names and permanent e-mail addresses. This allows us to contact contributors, e.g. for manuscripts about Avogadro development. This commit is currently signed as:

aouxwoux 81102668+aouxwoux@users.noreply.github.com

Which does neither. Please include a full name and permanent email address. Thanks.

[ghutchis]

Great - apologies for the delay. LGTM - many thanks.

[welcome]

Congrats on merging your first pull request! 🎉 Thanks for making Avogadro better for everyone!