Can´t use DeepSeek API

### Describe the bug

When I run the following command:

```bash
export COPILOT_PROVIDER_BASE_URL="https://api.deepseek.com" \
export COPILOT_PROVIDER_API_KEY="...." \
export COPILOT_PROVIDER_TYPE="openai" \
export COPILOT_MODEL="deepseek-v4-pro" \
copilot
```

When I make a question to the copilot, I get this answer back:

```
● Request failed (transient_bad_request). Retrying...
● Request failed (transient_bad_request). Retrying...
✗ 400 The `reasoning_content` in the thinking mode must be passed back to the API.
```

I confirm that the DeepSeek API needs the entire reasoning_content field to be returned back to the API every time. In my own code I had the same problem and could rapidly fix the issue by sending back the the entire conversation.

### Affected version

_No response_

### Steps to reproduce the behavior

I run the following command:

```bash
export COPILOT_PROVIDER_BASE_URL="https://api.deepseek.com" \
export COPILOT_PROVIDER_API_KEY="...." \
export COPILOT_PROVIDER_TYPE="openai" \
export COPILOT_MODEL="deepseek-v4-pro" \
copilot
```

Ensure to have a valid api key for the DeepSeek endpoint

### Expected behavior

I expect the request to be fulfilled.

### Additional context

_No response_

Empieza reproduciendo el problema a través del punto de entrada `copilot` con las variables de entorno de DeepSeek documentadas e inspecciona cómo se ensambla la conversación de la solicitud. La corrección estará terminada cuando se devuelva el `reasoning_content` completo a la API de DeepSeek y la solicitud se complete correctamente sin el error 400.

Escrito por el modelo de indexación a partir del texto del issue.