fix: ignore invalid gitignore bracket ranges

[Gares95]

Summary

This updates GitIgnoreSpecPattern so reversed invalid bracket ranges such as [z-a] and a[z-a] are treated as null patterns instead of propagating a regex compile error.

This matches the existing invalid-range handling direction for gitignore spec patterns and keeps GitIgnoreBasicPattern behavior unchanged.

What changed

• Validate completed bracket expressions in the range_error == "raise" path.
• Convert regex compile failures for invalid bracket ranges into the existing _RangeError flow.
• Update the focused Issue Different gitignore pattern interpretations between python-pathspec and Git #93 regression test so [z-a] and a[z-a] produce null patterns:
  • include is None
  • regex is None

Why

The surrounding test already notes that these patterns should become null patterns to fully replicate Git behavior. This makes reversed invalid ranges consistent with the existing invalid-range cases already covered by the test suite.

Tests

Ran:

python -m unittest tests.test_04_gitignore_spec.GitIgnoreSpecPatternTest.test_15_issue_93_c_2_invalid
python -m unittest tests.test_04_gitignore_spec
python -m unittest tests.test_03_gitignore_basic
python -m unittest tests.test_06_gitignore
python -m unittest discover -t . -s tests/

Full configured unittest result:

Ran 197 tests in 0.203s
OK (skipped=276)

Compatibility notes

• Scoped to GitIgnoreSpecPattern.
• GitIgnoreBasicPattern behavior is unchanged.
• Matching, path normalization, backend dispatch, and tree-walk behavior are unchanged.