Expand test coverage with mocked Copilot SDK

[trsdn]

Summary

Improve test coverage with comprehensive unit and integration tests using mocked dependencies.

Tasks

• Create mock for \CopilotClient\ and \Session\
• Add unit tests for \ormatLocalDate()\ and \getDateRange()\ utilities
• Add unit tests for \mergeDuplicates()\ function
• Add integration tests for API endpoints (/api/news-stream, /api/news, /api/summarize-topic)
• Add error case tests (network failures, parse errors, timeouts)
• Add test coverage reporting

Acceptance Criteria

• Test coverage > 80%
• All utility functions have unit tests
• API endpoints have integration tests with mocked responses
• CI runs tests on every PR