Merge pull request #2638 from pythonnet/no-stashing-embedding-tests

- Run most tests in a single Python instance, initialized at start and shutdown at the end
- Improve uv CI setup
- Fix python test runner by essentially replacing it with a call to pytest.main
- Improve the PythonDLL exception
- Run tests that require reinitialization of the PythonEngine only on .NET Framework/Mono, where we still have a working formatter for the stashing