- Add common SynthStationID helpers for coordinate-based providers
- Use shared helper for Open-Meteo and OpenWeather station ID synthesis
- Require both lat/lon when generating synthetic IDs to avoid misleading defaults
- Remove unused Open-Meteo normalizer wrapper code
This reduces cross-provider duplication while keeping provider-specific
mapping logic explicit and readable.
Add shared normalizer helpers to centralize payload extraction, JSON decoding,
and event finalization/validation.
Refactor NWS, Open-Meteo, and OpenWeather observation normalizers to use the
shared spine, removing repeated boilerplate while preserving provider-specific
mapping logic.