Test section whitespace defaults and validation

[ryanduguid]

The strip_section_whitespace tests cover successful stripping, but not preserving default names or rejecting names that become duplicate or empty. Add 16 cases across ASCII space and three Unicode spaces, covering constructor and parse() defaults, error messages and source lines.

Verification

• Built-wheel tests pass on Linux Python 3.10.22, 3.11.17, 3.12.14, 3.13.16, 3.14.7 and 3.15.0rc3: 70 tests per version. Windows Python 3.14.8 also passes all 70 tests against the built wheel.
• Complete pre-commit run --all-files passes on Linux: pyproject-fmt, Ruff checks and formatting, and mypy. Setuptools wheel and source distribution builds pass.
• All 54 original tests pass mutations that force stripping by default, permit duplicate normalised names or permit empty stripped names. The new cases reject them with 8, 4 and 4 failures respectively. Production source was restored after the checks.
• Whitespace checks pass. The Linux checked patch and local source match.

Scope and limitations

Only testing/test_iniconfig.py changes. The existing build warning about the missing tox.ini manifest entry remains. Windows Python versions other than 3.14, stable Python 3.15, macOS, benchmarks, packaging action attestations and upstream CI have not been verified.

[RonnyPfannschmidt]

deferring action on this as its a mechanic test addition without reiteration