Fall back when Python environments API is incomplete

[sama Pyb (Pybsama)]

Summary

• validate the preferred Python Environments API's required runtime capabilities before wrapping or caching it
• fall back to the legacy Python Extension API when the preferred API returns undefined or incomplete exports
• keep onDidChangePackages optional and provide a no-op disposable when that non-critical event is unavailable

Context

When the Python Environments integration is disabled, the installed extension can return undefined exports without throwing. The shared provider previously cached that result and later failed while subscribing to environment/package events instead of trying its existing legacy fallback.

This change belongs in the shared package consumed by Pylint and the other VS Code Python tool extensions. It does not bump the package version or update downstream submodules; the existing release workflow owns those steps after merge.

Addresses microsoft/vscode-pylint#856.

Testing

• TypeScript test suite: 134 passing
• TypeScript build
• ESLint
• Python test suite: 333 passing on Python 3.10
• Python flake8, Black, and isort checks
• version consistency validation