refactor: encapsulate server startup in static method

src/server.ts

@@ -71,7 +71,13 @@ export class WebhookServer {
   }
 }
 
-Bun.serve({
+static start() {
+  if (process.env.NODE_ENV !== 'test') {
+    const server = Bun.serve({
       port: PORT,
       fetch: WebhookServer.handleRequest,
     });
+    console.log(`Server running on port ${PORT}`);
+    return server;
+  }
+}