Refactor OpenWeather normalizers to improve structure and reuse by moving
provider-specific helper functions out of observation.go and into a new
common.go.
This keeps observation.go focused on schema matching and domain mapping,
preserves the “one normalizer per file” convention, and establishes a clear
home for helpers that will be shared by future OpenWeather forecast and alert
normalizers.
No functional behavior changes; this is a pure internal refactor.
