fix: silently discard unclosed bracket expressions

[ArmaanjeetSandhu]

Summary

Fixes #93 by making pathspec match Git's behavior for invalid gitignore patterns with unclosed bracket expressions.

Problem

Git silently discards patterns like [ and [!] (unclosed brackets), treating them as null-operations. Files with those literal names are tracked.

pathspec previously treated these as literal characters, creating regex patterns that would incorrectly ignore files named [ or [!].

Solution

Added validation to detect unclosed bracket expressions and return (None, None) (null-operation) instead of creating invalid regex patterns.

Changes:

• Added __has_unclosed_bracket() helper method to both GitIgnoreBasicPattern and GitIgnoreSpecPattern
• Modified pattern_to_regex() to check for unclosed brackets before processing
• Updated 4 tests (2 in each test file) that were documenting buggy behavior to now test correct behavior

Verification

Tested against git check-ignore to confirm identical behavior:

$ echo '[' > .gitignore
$ touch '['
$ git check-ignore '['
# (no output = file is tracked) ✓

With this fix, pathspec now produces the same result.

Test results:

• All tests pass after updating the 4 tests that documented the bug
• Pattern [ → files named [ are tracked ✓
• Pattern [!] → files named [!] are tracked ✓
• Valid patterns like [abc] still work correctly ✓

Related

This completes the fixes mentioned in issue #93. The other issues mentioned (foo**/bar and leading whitespace) were already fixed in v1.0.0.