test: improve test coverage and add comprehensive CLI tests

- Fix compilation errors in existing tests (WriteToFile parameter mismatch)
- Add comprehensive CLI command tests (cmd_test.go) covering URL parsing, date filtering, path generation
- Add integration tests (integration_test.go) for end-to-end command execution scenarios
- Improve lib test coverage from 89.7% to 92.4%
- Add tests for WriteToFile with addSourceURL functionality for all formats
- Add FetchURLs edge case tests (empty lists, single URL, context cancellation)
- Fix date filtering test flakiness with deterministic date assignment
- Add error handling tests for format conversions
- Enhance fetcher tests with additional edge cases and concurrency scenarios

Author: alexferrari88