Error on top-level options in pytest.toml

[vismaytiwari]

Closes #14638.

Summary

• Keep the [pytest] table required in pytest.toml and .pytest.toml.
• Raise a clear configuration error when either file contains top-level scalar options without a [pytest] table, instead of silently ignoring them.
• Continue allowing empty pytest.toml files.
• Add regression coverage and update the changelog.

Verification

• Manually reproduced the original behavior before the change: top-level addopts was silently ignored.
• Verified the updated behavior with a small project: top-level addopts = ["-v"] exits with code 4 and reports that configuration must be under [pytest].
• Verified an empty pytest.toml remains valid and the sample test passes.
• SETUPTOOLS_SCM_PRETEND_VERSION=9.2.0.dev0 uv run --no-project --with-editable . --with '.[dev]' --python 3.11 python -m pytest testing/test_config.py -q (256 passed, 1 xfailed)
• uvx --from pre-commit==4.4.0 pre-commit run --files src/_pytest/config/findpaths.py testing/test_config.py doc/en/reference/customize.rst changelog/14638.bugfix.rst