fix: lint errors - use modules/json, http.MethodPost, remove unused func

cmd/mcp-server/main.go

@@ -25,7 +25,7 @@ package main
 import (
 	"bufio"
 	"bytes"
-	"encoding/json"
+	"code.gitea.io/gitea/modules/json"
 	"flag"
 	"fmt"
 	"io"
@@ -111,7 +111,7 @@ func main() {
 func forwardToGitea(request []byte) ([]byte, error) {
 	mcpURL := giteaURL + "/api/v2/mcp"
 
-	req, err := http.NewRequest("POST", mcpURL, bytes.NewReader(request))
+	req, err := http.NewRequest(http.MethodPost, mcpURL, bytes.NewReader(request))
 	if err != nil {
 		return nil, fmt.Errorf("create request: %w", err)
 	}