fix: safely access max_output_tokens in payload to prevent runtime errors

Author: cuipengfei

src/routes/chat-completions/handler.ts

@@ -31,7 +31,7 @@ export async function handleCompletion(c: Context) {
 
     payload = {
       ...payload,
-      max_tokens: selectedModel?.capabilities.limits.max_output_tokens,
+      max_tokens: selectedModel?.capabilities?.limits?.max_output_tokens,
     }
     consola.debug("Set max_tokens to:", JSON.stringify(payload.max_tokens))
   }