Replace io.ReadAll + json.Unmarshal with streaming JSON decoder

Refactor the internal HTTP helper from get() returning raw bytes to
getAndDecode() that streams directly into the target struct via
json.NewDecoder. This eliminates the intermediate []byte allocation
from io.ReadAll on every API response.

The new decoder also validates that responses contain exactly one JSON
value by attempting a second Decode after the primary one — any content
beyond the first value (e.g., concatenated objects from a misbehaving
proxy) returns an error instead of silently discarding it.

Changes:
- api/client.go: Replace get() with getAndDecode(), update FetchStores
  and FetchSavings callers to use the new signature
- api/client_test.go: Add TestFetchSavings_TrailingJSONIsRejected and
  TestFetchStores_MalformedJSONReturnsDecodeError covering the new
  decoder error paths

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/client.go

@@ -3,6 +3,7 @@ package api
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -42,10 +43,10 @@ func NewClientWithBaseURLs(savingsURL, storeURL string) *Client {
 	}
 }
 
-func (c *Client) get(ctx context.Context, reqURL, storeNumber string) ([]byte, error) {
+func (c *Client) getAndDecode(ctx context.Context, reqURL, storeNumber string, out any) error {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, reqURL, nil)
 	if err != nil {
-		return nil, fmt.Errorf("creating request: %w", err)
+		return fmt.Errorf("creating request: %w", err)
 	}
 
 	req.Header.Set("Accept", "application/json")
@@ -56,19 +57,22 @@ func (c *Client) get(ctx context.Context, reqURL, storeNumber string) ([]byte, e
 
 	resp, err := c.httpClient.Do(req)
 	if err != nil {
-		return nil, fmt.Errorf("executing request: %w", err)
+		return fmt.Errorf("executing request: %w", err)
 	}
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("unexpected status %d from %s", resp.StatusCode, reqURL)
+		return fmt.Errorf("unexpected status %d from %s", resp.StatusCode, reqURL)
 	}
 
-	body, err := io.ReadAll(resp.Body)
+	dec := json.NewDecoder(resp.Body)
-	if err != nil {
+	if err := dec.Decode(out); err != nil {
-		return nil, fmt.Errorf("reading response: %w", err)
+		return fmt.Errorf("decoding response: %w", err)
 	}
-	return body, nil
+	if err := dec.Decode(new(struct{})); !errors.Is(err, io.EOF) {
+		return fmt.Errorf("decoding response: trailing JSON content")
+	}
+	return nil
 }
 
 // FetchStores finds Publix stores near the given zip code.
@@ -81,14 +85,9 @@ func (c *Client) FetchStores(ctx context.Context, zipCode string, count int) ([]
 		"zipCode":                  {zipCode},
 	}
 
-	body, err := c.get(ctx, c.storeURL+"?"+params.Encode(), "")
-	if err != nil {
-		return nil, fmt.Errorf("fetching stores: %w", err)
-	}
-
 	var resp StoreResponse
-	if err := json.Unmarshal(body, &resp); err != nil {
+	if err := c.getAndDecode(ctx, c.storeURL+"?"+params.Encode(), "", &resp); err != nil {
-		return nil, fmt.Errorf("decoding stores: %w", err)
+		return nil, fmt.Errorf("fetching stores: %w", err)
 	}
 	return resp.Stores, nil
 }
@@ -104,14 +103,9 @@ func (c *Client) FetchSavings(ctx context.Context, storeNumber string) (*Savings
 		"getSavingType":            {"WeeklyAd"},
 	}
 
-	body, err := c.get(ctx, c.savingsURL+"?"+params.Encode(), storeNumber)
-	if err != nil {
-		return nil, fmt.Errorf("fetching savings: %w", err)
-	}
-
 	var resp SavingsResponse
-	if err := json.Unmarshal(body, &resp); err != nil {
+	if err := c.getAndDecode(ctx, c.savingsURL+"?"+params.Encode(), storeNumber, &resp); err != nil {
-		return nil, fmt.Errorf("decoding savings: %w", err)
+		return nil, fmt.Errorf("fetching savings: %w", err)
 	}
 	return &resp, nil
 }

internal/api/client_test.go

@@ -127,6 +127,34 @@ func TestFetchStores_NoResults(t *testing.T) {
 	assert.Empty(t, result)
 }
 
+func TestFetchSavings_TrailingJSONIsRejected(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"Savings":[],"LanguageId":1} {"extra":true}`))
+	}))
+	defer srv.Close()
+
+	client := api.NewClientWithBaseURLs(srv.URL, "")
+	_, err := client.FetchSavings(context.Background(), "1425")
+
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "decoding")
+}
+
+func TestFetchStores_MalformedJSONReturnsDecodeError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"Stores":`))
+	}))
+	defer srv.Close()
+
+	client := api.NewClientWithBaseURLs("", srv.URL)
+	_, err := client.FetchStores(context.Background(), "37042", 5)
+
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "decoding")
+}
+
 func TestStoreNumber(t *testing.T) {
 	tests := []struct {
 		input string