Add Publix API client for savings and store location endpoints

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.

internal/api/client_test.go

@@ -0,0 +1,143 @@
+package api_test
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/tayloree/publix-deals/internal/api"
+)
+
+func ptr(s string) *string { return &s }
+
+func newTestSavingsServer(t *testing.T, storeNumber string, items []api.SavingItem) *httptest.Server {
+	t.Helper()
+	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Verify the PublixStore header is sent
+		got := r.Header.Get("PublixStore")
+		if storeNumber != "" {
+			assert.Equal(t, storeNumber, got, "PublixStore header mismatch")
+		}
+
+		resp := api.SavingsResponse{
+			Savings:    items,
+			LanguageID: 1,
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+}
+
+func newTestStoreServer(t *testing.T, stores []api.Store) *httptest.Server {
+	t.Helper()
+	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.NotEmpty(t, r.URL.Query().Get("zipCode"), "zipCode param required")
+
+		resp := api.StoreResponse{Stores: stores}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+}
+
+func TestFetchSavings(t *testing.T) {
+	items := []api.SavingItem{
+		{
+			ID:             "test-1",
+			Title:          ptr("Chicken Breasts"),
+			Savings:        ptr("$3.99 lb"),
+			Department:     ptr("Meat"),
+			Categories:     []string{"meat"},
+			StartFormatted: "2/18",
+			EndFormatted:   "2/24",
+		},
+		{
+			ID:         "test-2",
+			Title:      ptr("Nutella"),
+			Savings:    ptr("Buy 1 Get 1 FREE"),
+			Categories: []string{"bogo", "grocery"},
+		},
+	}
+
+	srv := newTestSavingsServer(t, "1425", items)
+	defer srv.Close()
+
+	client := api.NewClientWithBaseURLs(srv.URL, "")
+	resp, err := client.FetchSavings(context.Background(), "1425")
+
+	require.NoError(t, err)
+	assert.Len(t, resp.Savings, 2)
+	assert.Equal(t, "Chicken Breasts", *resp.Savings[0].Title)
+	assert.Equal(t, "Buy 1 Get 1 FREE", *resp.Savings[1].Savings)
+}
+
+func TestFetchSavings_EmptyStore(t *testing.T) {
+	srv := newTestSavingsServer(t, "", nil)
+	defer srv.Close()
+
+	client := api.NewClientWithBaseURLs(srv.URL, "")
+	resp, err := client.FetchSavings(context.Background(), "")
+
+	require.NoError(t, err)
+	assert.Empty(t, resp.Savings)
+}
+
+func TestFetchSavings_ServerError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer srv.Close()
+
+	client := api.NewClientWithBaseURLs(srv.URL, "")
+	_, err := client.FetchSavings(context.Background(), "1425")
+
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "500")
+}
+
+func TestFetchStores(t *testing.T) {
+	stores := []api.Store{
+		{Key: "01425", Name: "Peachers Mill", City: "Clarksville", State: "TN", Zip: "37042", Distance: "5"},
+		{Key: "00100", Name: "Downtown", City: "Nashville", State: "TN", Zip: "37201", Distance: "15"},
+	}
+
+	srv := newTestStoreServer(t, stores)
+	defer srv.Close()
+
+	client := api.NewClientWithBaseURLs("", srv.URL)
+	result, err := client.FetchStores(context.Background(), "37042", 5)
+
+	require.NoError(t, err)
+	assert.Len(t, result, 2)
+	assert.Equal(t, "Peachers Mill", result[0].Name)
+	assert.Equal(t, "01425", result[0].Key)
+}
+
+func TestFetchStores_NoResults(t *testing.T) {
+	srv := newTestStoreServer(t, nil)
+	defer srv.Close()
+
+	client := api.NewClientWithBaseURLs("", srv.URL)
+	result, err := client.FetchStores(context.Background(), "00000", 5)
+
+	require.NoError(t, err)
+	assert.Empty(t, result)
+}
+
+func TestStoreNumber(t *testing.T) {
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"01425", "1425"},
+		{"00100", "100"},
+		{"1425", "1425"},
+		{"0", ""},
+	}
+	for _, tt := range tests {
+		assert.Equal(t, tt.want, api.StoreNumber(tt.input), "StoreNumber(%q)", tt.input)
+	}
+}