Support ms-python.vscode-python-envs for Python environment discovery

[dv-ar]

Summary

• Adds support for the Python Environments extension (ms-python.vscode-python-envs) as a Python discovery source, tried before ms-python.python
• Prefers the active workspace environment, falls back to the global active environment, then includes all other discovered environments
• Skips broken environments (those with an error field) before spawning subprocesses
• Subscribes to onDidChangeEnvironment and onDidChangeEnvironments with a shared 500 ms debounce to re-validate Jupytext when the active environment changes
• Updates README and pythonExecutable setting description to document all three discovery sources in priority order
• Bumps version to 1.4.5

Test plan

• With ms-python.vscode-python-envs installed: Output → Jupytext Sync shows ms-python.vscode-python-envs: resolved N candidate path(s): ...
• With only ms-python.python installed: fallback works as before
• With neither extension installed: python/python3 PATH fallback works
• Change active Python environment in the Python Environments UI → Jupytext re-validates automatically
• Broken environments (with error field) are skipped and not probed

🤖 Generated with Claude Code

[caenrigen]

Please elaborate first on the problem(s) this PR is intended to solve.

I am currently using Jupytext from a virtualenv and it works. What are the scenarios that don't work for you?