### Check for existing issues

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

### What happened?

When sending a request to a **multipart/form-data** endpoint (e.g. `POST /v1/images/edits`) with a `user_config` form field whose value is a JSON string, the proxy does **not** JSON-decode the field. As a result `user_config` arrives downstream as a `str` rather than a `dict`, which then either:

- crashes with `litellm.router.Router() argument after ** must be a mapping, not str` at `route_llm_request.py:200` (`Router(**user_config)`), or - gets silently dropped during request setup, after which `route_request` falls through to the catch-all and raises `ProxyModelNotFoundError("Invalid model name passed in model=...")` at `route_llm_request.py:324`,

depending on the exact request path.

The same `user_config` payload works correctly when sent on a JSON-bodied endpoint (e.g. `POST /v1/images/generations`), because `_read_request_body` parses the entire JSON body and `user_config` arrives as a `dict`.

**Root cause** — `litellm/proxy/common_utils/http_parsing_utils.py`, in the multipart branch of `_read_request_body`:

```python if "form" in co…