Add Test Infrastructure for License Download Functionality

Problem

Currently, there's no way to test critical functionality like license download failures, atomic updates, and error handling without manually breaking production systems or waiting for real-world failures. Mainly to test the change implemented in #141

Proposed Solution

Implement a proper test infrastructure with:

1. License Download Testing Ability to simulate individual license download failures Test atomic update behavior when some downloads fail Verify progress reporting and error handling
2. UI Testing Test download status display in options page Verify failed download warnings and success messages Test permission error scenarios

Benefits

• Confidence in critical fault-tolerance features
• Easier debugging of edge cases
• Better development workflow for future license-related features
• Reduced risk when deploying updates