fix: throw error if fetch called before build

added a check in the fetch getter to prevent its usage before the application is built, ensuring proper usage and avoiding runtime issues.

app/src/App.ts

@@ -244,6 +244,10 @@ export class App<
    }
 
    get fetch(): Hono["fetch"] {
+      if (!this.isBuilt()) {
+         throw new Error("App is not built yet, run build() first");
+      }
+
       return this.server.fetch as any;
    }