## Bug Description

When `use_chat_completions_api: true` is configured on a model with `model: openai/...`, the proxy correctly calls the upstream OpenAI API's `/v1/chat/completions` endpoint. However, when converting the response back to Anthropic Messages API format, the `content` field is dropped when the upstream also returns `reasoning_content` (a non-standard field used by DeepSeek, Kimi, etc. for chain-of-thought).

## Expected

The Anthropic-format response should include the model text content: ``` {"type":"message","content":[{"type":"text","text":"hello"}],"stop_reason":"end_turn"} ```

## Actual

Response has empty content when upstream includes reasoning: ``` {"type":"message","content":[],"stop_reason":"end_turn"} ```

## Steps

1. Config: ```yaml model_list: - model_name: deepseek-v4-flash litellm_params: model: openai/deepseek-v4-flash api_base: https://opencode.ai/zen/go/v1 api_key: os.environ/OPENCODE_GO_API_KEY use_chat_completions_api: true ```

2. Start proxy: `litellm --config config.yaml --port 4000`

3. Send request via `/v1/messages` (Anthropic): `curl http://localhost:4000/v1/messages -H "Authorization: Bearer sk-test" -H "an…