Migrate pytest configs to native TOML and enable strict mode

[correctmost]

PR Description:

Native TOML configs and strict mode were added in pytest 9.0:

• https://docs.pytest.org/en/stable/changelog.html#pytest-9-0-0-2025-11-05
• https://docs.pytest.org/en/stable/explanation/goodpractices.html#strict-mode