* test: add comprehensive unit tests for Context.File() method
- Add TestContextFile with multiple test scenarios in context_test.go
- Add simplified tests in context_file_test.go
- Cover file serving, 404 handling, directory access, HEAD requests, and Range requests
- All tests pass successfully
* fix: resolve gci formatting issues in test files
* fix: correct test file paths and add test file to gin directory
* move test_file.txt to testdata directory as suggested by maintainer
* fix: update test file paths to use testdata directory
