[BUG] Endpoints with Form Params not handled correctly

[davidschwab98]

Describe the bug

Currently fastapi_mcp doesn't correctly handles endpoints with form paramers. The reason for this is that in the _request method of the FastApiMCP class, all parameters are passed as the json argument, wich leads to a 422 return (should be the body param instead).

To Reproduce
Create an enpoint with form parameters.

System Info
Python 3.13, latest versions of FastAPI and fastapi_mcp