perf(json): replace encoding/json with bytedance/sonic

Migrate all JSON marshaling and unmarshaling operations to use
github.com/bytedance/sonic for improved performance. This affects
adapters, middleware, proxy handlers, and the sync store.

test/go.mod

@@ -5,6 +5,6 @@ go 1.24.5
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/stretchr/testify v1.11.1 // indirect
+	github.com/stretchr/testify v1.11.1
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

test/integration_test.go

@@ -4,13 +4,13 @@ package integration
 
 import (
 	"bytes"
-	"encoding/json"
 	"io"
 	"net/http"
 	"os"
 	"testing"
 	"time"
 
+	"github.com/bytedance/sonic"
 	"github.com/stretchr/testify/require"
 )
 
@@ -54,7 +54,7 @@ func TestEndToEnd(t *testing.T) {
 }
 
 func postJSONWithAuth[T any](t *testing.T, client *http.Client, url string, body interface{}, out T, token string) T {
-	b, err := json.Marshal(body)
+	b, err := sonic.Marshal(body)
 	require.NoError(t, err)
 
 	req, err := http.NewRequest(http.MethodPost, url, bytes.NewReader(b))
@@ -76,7 +76,7 @@ func postJSONWithAuth[T any](t *testing.T, client *http.Client, url string, body
 	if out != nil {
 		data, _ := io.ReadAll(resp.Body)
 		if len(data) > 0 {
-			require.NoError(t, json.Unmarshal(data, out))
+			require.NoError(t, sonic.Unmarshal(data, out))
 		}
 	}
 	return out

test/mock-upstream/main.go

@@ -1,9 +1,10 @@
 package main
 
 import (
-	"encoding/json"
 	"log"
 	"net/http"
+
+	"github.com/bytedance/sonic"
 )
 
 func main() {
@@ -30,7 +31,7 @@ func main() {
 			},
 		}
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(resp)
+		_ = sonic.ConfigDefault.NewEncoder(w).Encode(resp)
 	})
 
 	mux.HandleFunc("/v1/models", func(w http.ResponseWriter, r *http.Request) {
@@ -41,7 +42,7 @@ func main() {
 			},
 		}
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(resp)
+		_ = sonic.ConfigDefault.NewEncoder(w).Encode(resp)
 	})
 
 	log.Println("mock-upstream listening on :8082")