### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

**问题描述：** 在 AI 客户端通过 LiteLLM 调用下游 MCP 服务端 https://xxx/mcp 时，若请求 Payload（如 HTML 报表）较大（超过 4096 字节），下游会频繁返回 500 Internal Server Error 导致调用失败。

**报错日志（LiteLLM 侧记录的下游 500）：** ``` litellm-1 | 11:34:32 - LiteLLM:ERROR: server.py:4022 - Error handling MCP request: 'utf-8' codec can't decode byte 0xe9 in position 4095: unexpected end of data litellm-1 | Traceback (most recent call last): litellm-1 | File "/app/.venv/lib/python3.13/site-packages/litellm/proxy/_experimental/mcp_server/server.py", line 3811, in handle_streamable_http_mcp litellm-1 | is_initialize = _is_initialize_request(body) litellm-1 | File "/app/.venv/lib/python3.13/site-packages/litellm/proxy/_experimental/mcp_server/server.py", line 3196, in _is_initialize_request litellm-1 | data = json.loads(body) litellm-1 | File "/usr/lib/python3.13/json/__init__.py", line 347, in loads litellm-1 | s = s.decode(detect_encoding(s), 'surrogatepass') litellm-1 | UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 4095: unexpected end of data litellm-1 | I…