pytest.toml: treat headerless top-level keys as pytest config

[MD-Mushfiqur123]

Closes #14638

Problem

When a pytest.toml or .pytest.toml file contains top-level keys without a [pytest] section header, those keys are silently ignored. This is surprising because:

• TOML sections are optional (unlike INI files)
• The filename itself already identifies the file as pytest configuration

Solution

Treat top-level non-table keys in pytest.toml/.pytest.toml as pytest configuration, as if they were written under [pytest]. If a [pytest] section is also present, its values are merged on top (taking precedence).

For example, this now works:

minversion = "9.0"
addopts = ["-v"]

Testing

• All existing TOML config tests pass (24 tests)
• Added test_headerless_pytest_toml for both pytest.toml and .pytest.toml
• Verified manually that the config values are properly parsed and applied

[RonnyPfannschmidt]

Ai slop when there's a correct pr existing