## Description

When using LiteLLM's Responses API (`/v1/responses`) to proxy Anthropic models, multi-step tool call flows produce:

``` text part chatcmpl-<id> not found ```

The AI SDK tracks text parts via `text-start` → `text-delta` → `text-end` by ID. During step 2+ (text after tool use), the `text-delta` event arrives with a `chatcmpl-` response-level ID that was never registered via `text-start`, causing the AI SDK to drop the text.

Single-step text-only responses work fine. Only multi-step tool call sequences break.

**The Bug**

In _transform_chat_completion_chunk_to_response_api_chunk:

``` if self._cached_item_id is None and chunk.id: self._cached_item_id = chunk.id # ← sets cache to "chatcmpl-aaa" on first chunk item_id = self._cached_item_id or chunk.id # ← uses cached value for all events

``` Step 1 (tool call): First chunk arrives, _cached_item_id is None, so it caches chunk.id = "chatcmpl-aaa". The _ensure_output_item_for_chunk method then overwrites it to "msg_{uuid}" for the output item. Text events use "msg_{uuid}" — consistent. ✅

Step 2 (text after tool result): LiteLLM internally makes a new Chat Completion call, which returns ch…