Add Unit Tests for `find_sponsor_page` Method

[DiegoPisa2003]

This PR adds comprehensive unit tests for the find_sponsor_page method in the DownloadSponsors class, along with an improvement to the fragment link detection logic.

Changes Made

Enhanced Fragment Link Detection (app/lib/download_sponsors.rb)

• Improved the logic for identifying fragment links by properly parsing URIs and comparing host/path components
• This prevents false positives when looking for sponsor links that might contain "#" in their URLs

Comprehensive Test Coverage (test/lib/download_sponsors_test.rb)

• Added a new test file with 7 test cases covering various scenarios

Testing Strategy

The tests use a local WEBrick server to simulate real HTTP responses, allowing us to test the actual HTML parsing logic without external dependencies. This approach provides reliable, fast test execution while maintaining realistic test conditions.

Note on extract_and_save_sponsors_data Testing

I intentionally did not add tests for the extract_and_save_sponsors_data method for the following reasons:

1. OpenAI API Integration: This method makes calls to the OpenAI API, which would require careful stubbing to prevent actual API calls during testing
2. External Gem Testing: The method relies on external gems that should already have their own test coverage from their maintainers

The current test coverage focuses on the core HTML parsing and link detection logic.