Improve test coverage to 100% with comprehensive context and edge case testing

[Copilot]

This PR significantly improves the test coverage of the waitfor-fs library from 88.9% to 100%, exceeding the required 80% threshold.

What was improved

The main gap in test coverage was in the Test() function's context error handling. The existing tests didn't cover the scenario where a context is already canceled or has exceeded its deadline before the file check is performed.

Changes made

Added 4 new comprehensive test cases:

1. TestFile_ContextCanceled - Tests behavior when a context is canceled before calling Test()
2. TestFile_ContextDeadlineExceeded - Tests behavior when a context deadline has already passed
3. TestFile_EmptyPath - Tests edge case with empty file path (file://)
4. TestFile_NonFileScheme - Tests behavior with non-file URL schemes

Also updated .gitignore to exclude coverage artifacts (coverage.html).

Coverage improvement

• Before: 88.9% coverage (3/4 statements in Test() function)
• After: 100.0% coverage (all statements covered)

The previously uncovered code path was:

if ctx.Err() != nil {
    return ctx.Err()
}

Validation

• All 8 tests pass with race detection enabled
• No linting issues (revive + go vet)
• Full CI pipeline compatibility maintained
• No breaking changes to existing functionality
• No bugs found in existing implementation

The library now has comprehensive test coverage that validates all code paths including context handling, error scenarios, and edge cases while maintaining full backward compatibility.

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