This change exposes the URLStreamHandlerFactory functionality from coursier
to the Java API, allowing users to customize URL handling for artifacts and repositories.

Key changes:
- Add withCustomHandlerFactory() method to Cache class
- Update ApiHelper to pass custom handler factory to underlying coursier cache
- Add comprehensive tests for the new functionality
- Update forked coursier version to 2.1.30-test

This enables support for custom URL schemes, authentication mechanisms,
and network behaviors in coursier operations.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Add comprehensive functional test that actually resolves artifacts using a custom
protocol handler, demonstrating real-world usage of URLStreamHandlerFactory support.

Key improvements:
- Implement working testproto:// protocol handler serving JAR, POM, and metadata files
- Create minimal valid JAR content with proper ZIP format
- Test actual artifact resolution through custom protocol
- Verify protocol integration by tracking URL requests
- Handle checksum files to support complete resolution flow

The test proves URLStreamHandlerFactory integration works correctly by showing
coursier successfully uses custom protocols for artifact downloads, as evidenced
by the cache path containing the custom protocol scheme.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Add comprehensive functional test that actually resolves artifacts using a custom
protocol handler, demonstrating real-world usage of URLStreamHandlerFactory support.

Key improvements:
- Implement working testproto:// protocol handler serving JAR, POM, and metadata files
- Create minimal valid JAR content with proper ZIP format
- Test actual artifact resolution through custom protocol
- Verify protocol integration by tracking URL requests
- Handle checksum files to support complete resolution flow

The test proves URLStreamHandlerFactory integration works correctly by showing
coursier successfully uses custom protocols for artifact downloads, as evidenced
by the cache path containing the custom protocol scheme.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

- Add section explaining use-forked-coursier branch
- Document build process for 2.1.30-SNAPSHOT
- Explain why this fork exists and key differences
- Include instructions for Java version requirements