Handle brackets in collection file paths

[hemanshu406]

• Improved test collection to correctly handle existing Python file paths containing square brackets.
• Updated collection path parsing to distinguish literal filenames from test-selection syntax.
• Added a regression test covering a file named test_[helper].py.

Testing

• python -m pytest testing/test_main.py -q
• 25 passed

Checklist

• Added regression test coverage.
• Verified the relevant tests pass.
• Added the required changelog fragment.
• Updated AUTHORS if required.
• Verified AI assistance is credited in commit trailers.

AI Assistance

AI tools assisted with implementation and testing. The contributor is responsible for reviewing and maintaining the changes.