### Check for existing issues

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

### What happened?

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## ISSUE 1 — Bug (primary)

**Title:** `[Bug]: KeyError: 'message' on Mistral chat.multi_completion responses (hosted tools like image_generation)`

## Check for existing issues

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

## What happened?

`litellm.completion` / `litellm.acompletion` raises `KeyError: 'message'` whenever Mistral returns a `chat.multi_completion` response.

Mistral's server-side / hosted tools on the chat completions endpoint (e.g. `image_generation`, connectors, `web_search`) do **not** return the OpenAI-standard choice shape. Instead of `object: "chat.completion"` with `choices[].message` (singular), Mistral returns:

- `object: "chat.multi_completion"` - each choice carries `messages` (**plural** — an array of turn messages: assistant tool-call turn, tool-result turn, final assistant turn) instead of `message` (singular).

litellm's response converter assumes the singular OpenAI shape and dies with a bare `KeyError: 'message'`.

**Expected behavior:** litellm should parse th…