fix(Add model api): Add VolcEngine to create api_key format error (#2490)

### What problem does this PR solve?


Add VolcEngine to create api_key format error
When constructing the json string, there was an extra "," at the end,
which caused a formatting error. This commit fixed the problem.


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/llm_app.py

         # Assemble ark_api_key endpoint_id into api_key
         llm_name = req["llm_name"]
         api_key = '{' + f'"ark_api_key": "{req.get("ark_api_key", "")}", ' \
-                        f'"ep_id": "{req.get("endpoint_id", "")}", ' + '}'
+                        f'"ep_id": "{req.get("endpoint_id", "")}" ' + '}'
     elif factory == "Tencent Hunyuan":
         api_key = '{' + f'"hunyuan_sid": "{req.get("hunyuan_sid", "")}", ' \
                         f'"hunyuan_sk": "{req.get("hunyuan_sk", "")}"' + '}'