HTTP client that wraps the Publix services API with two endpoints:
- /api/v4/savings — fetches weekly ad deals for a given store number
- /api/v1/storelocation — finds nearby stores by ZIP code
Includes request types (SavingsResponse, SavingItem, StoreResponse,
Store) mapping directly to the Publix JSON schema. The client sends
a PublixStore header for store-scoped requests and uses a 15-second
timeout. Tests use httptest servers to verify header propagation,
JSON decoding, and error handling for non-200 responses.
