test(aggregators): add comprehensive test suite for Kagi News aggregator
Adds 57 tests with 83% code coverage across all components:
Test coverage by component:
- RSS Fetcher (5 tests): fetch, retry, timeout, invalid XML
- HTML Parser (8 tests): all sections, missing sections, full story
- Rich Text Formatter (10 tests): facets, UTF-8, multi-byte chars
- State Manager (12 tests): deduplication, rolling window, persistence
- Config Manager (3 tests): YAML validation, env vars
- Main Orchestrator (9 tests): E2E flow, error isolation, dry-run
- E2E Tests (6 skipped): require live Coves API
Test results: 57 passed, 6 skipped, 1 warning in 8.76s
Fixtures:
- Real Kagi News RSS item with all sections (sample_rss_item.xml)
- Used to validate parser against actual feed structure
All tests use pytest with mocking for HTTP requests (responses library).