Respect file extensions with --filter-files

[yuefdev]

Fixes #1984.

--filter-files applies skip rules to explicitly supplied files but ignores supported extensions. Consequently, isort example.py --filter-files --ext txt --check still checks the Python file and can exit with an error.

Apply the existing is_supported_filetype check to file arguments during filtering. Directories continue through normal discovery, and explicitly supplied files without --filter-files retain their existing behaviour.

Regression tests cover explicit files, directory discovery, check mode, and the unfiltered case. Two new cases fail before the fix.

Validation:

• python -m pytest tests/unit/test_main.py tests/unit/test_settings.py -q --tb=short --show-capture=no: 57 passed, 1 skipped.
• Ruff lint/format and mypy isort/main.py passed.

AI assistance: Codex prepared this fix and regression coverage and ran the checks above. No independent human review is claimed.