test: improve weather_client_alerts.py (AlertAggregator) coverage from 11% to 90%+

[Orinks]

Problem

weather_client_alerts.py has only 11% test coverage. The AlertAggregator class handles critical alert deduplication logic that is essentially untested.

Uncovered functionality

• aggregate_alerts(): combining NWS + VC alerts, source tagging, empty handling
• _deduplicate_alerts(): grouping logic
• _is_duplicate(): event matching, area overlap, timezone-aware/naive onset comparison
• _areas_overlap(): normalization and set intersection
• _merge_duplicate_alerts(): source priority, description merging, metadata preference

Solution

Add unit tests covering all methods and edge cases.

Acceptance Criteria

• Coverage reaches 90%+
• All existing tests continue to pass

[Orinks]

Covered by PR #263