## Description

When a Responses API request contains an assistant message with BOTH text and `function_call` content items, the `function_call` is not properly converted to the Chat Completions `tool_calls` array during the Responses→Chat transformation. This causes a "Missing corresponding tool call for tool response message" error when the corresponding `function_call_output` is processed.

## Root Cause

In the Responses API → Chat Completions request transformation, mixed-content assistant messages (those containing both `text` and `function_call` content items) are not properly handled. The `function_call` content item appears to be stripped without being converted to a `tool_calls` entry, leaving orphan `function_call_output` tool results that fail validation in downstream providers (especially Gemini).

## Error

``` Missing corresponding tool call for tool response message. Received - message={'role': 'tool', 'content': 'result', 'tool_call_id': 'call_good'}, last_message_with_tool_calls={'role': 'assistant', 'content': [{'type': 'text', 'text': 'ok'}]} ```

## Steps to Reproduce

Send a `/v1/responses` request with this `input`:

```json { "model": "gpt-5.3-codex", …