### Check for existing issues

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

### What happened?

Forcing a named tool call through `/v1/responses` (`tool_choice: {"type": "function", "name": "..."}`) with `stream: true` crashes with a 500 on any model that falls back to the `litellm_completion` bridge - Bedrock and Anthropic in particular, since neither has a native Responses API integration.

Expected: the request should stream back a normal response using the forced tool.

Root cause: `ResponsesAPIResponse.tool_choice` only accepts a string (`none`/`auto`/`required`) or the flat Responses API shape `{"type": "function", "name": "..."}`, but the `litellm_completion` bridge was populating it with Chat Completion's nested shape `{"type": "function", "function": {"name": "..."}}` instead.

### Steps to Reproduce

1. Start the proxy pointed at an Anthropic or Bedrock model 2. Send a `/v1/responses` request with `stream: true`, a `tools` list, and `tool_choice` set to a specific function name 3. Observe the 500 error on the first SSE event```bash ``` curl -sN -X POST http://localhost:4000/v1/responses \ -H "Authorization: Bearer sk-1234" \ …