feat(api, watcher): add zstd decoding for request logs and payload diff support

- Added `zstd` decoding support in request logging, including helper functions to process `Content-Encoding` headers.
- Enhanced config diff logic to compare payload-specific rules and track changes in payload configurations.
- Added tests to validate `zstd` decoding and payload diff behavior.

internal/api/middleware/request_logging_test.go

@@ -1,11 +1,16 @@
 package middleware
 
 import (
+	"bytes"
 	"io"
 	"net/http"
+	"net/http/httptest"
 	"net/url"
 	"strings"
 	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/klauspost/compress/zstd"
 )
 
 func TestShouldSkipMethodForRequestLogging(t *testing.T) {
@@ -136,3 +141,43 @@ func TestShouldCaptureRequestBody(t *testing.T) {
 		}
 	}
 }
+
+func TestCaptureRequestInfoDecodesZstdRequestBodyForLog(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	payload := []byte(`{"model":"test-model","stream":true}`)
+	var compressed bytes.Buffer
+	encoder, errNewWriter := zstd.NewWriter(&compressed)
+	if errNewWriter != nil {
+		t.Fatalf("zstd.NewWriter: %v", errNewWriter)
+	}
+	if _, errWrite := encoder.Write(payload); errWrite != nil {
+		t.Fatalf("zstd write: %v", errWrite)
+	}
+	if errClose := encoder.Close(); errClose != nil {
+		t.Fatalf("zstd close: %v", errClose)
+	}
+	compressedBytes := compressed.Bytes()
+
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	req := httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(compressedBytes))
+	req.Header.Set("Content-Encoding", "zstd")
+	c.Request = req
+
+	info, errCapture := captureRequestInfo(c, true)
+	if errCapture != nil {
+		t.Fatalf("captureRequestInfo: %v", errCapture)
+	}
+	if !bytes.Equal(info.Body, payload) {
+		t.Fatalf("logged request body = %q, want %q", string(info.Body), string(payload))
+	}
+
+	restoredBody, errRead := io.ReadAll(c.Request.Body)
+	if errRead != nil {
+		t.Fatalf("read restored request body: %v", errRead)
+	}
+	if !bytes.Equal(restoredBody, compressedBytes) {
+		t.Fatal("request body was not restored with the original compressed bytes")
+	}
+}