Using multiple roles with the R1 model.

[Vein05]

Currently the api returns a 400 error when trying to use multiple roles and contents with the r1 model.

	request := &deepseek.ChatCompletionRequest{
		Model: deepseek.DeepSeekReasoner,
		Messages: []deepseek.ChatCompletionMessage{
			{Role: constants.ChatMessageRoleUser, Content: "How are you?"},
			{Role: constants.ChatMessageRoleSystem, Content: "Talk like an anime girl."},
		},
		Temperature: 1.0,
		ResponseFormat: &deepseek.ResponseFormat{
			Type: "text",
		},
	}

Find out the reason why, and update the docs as needed!

[Vein05]

There are some constraints about using the new R-1 model.

Constraints:
First Message: The first message (excluding system messages) must be a user message. The API does not accept an assistant message as the first message.

Last Message: The last message must be a user message or an assistant message with prefix mode enabled.

Correct Usage Examples:

1. Using System and User Messages:
   If you want to include a system message, it should be the first message, followed by a user message.

request := &deepseek.ChatCompletionRequest{
    Model: deepseek.DeepSeekReasoner,
    Messages: []deepseek.ChatCompletionMessage{
        {Role: constants.ChatMessageRoleSystem, Content: "You are a helpful assistant."},
        {Role: constants.ChatMessageRoleUser, Content: "How are you?"},
    },
}

2. Using User Messages Only:
   If you only need to use user messages, ensure that the last message is from the user.

request := &deepseek.ChatCompletionRequest{
    Model: deepseek.DeepSeekReasoner,
    Messages: []deepseek.ChatCompletionMessage{
        {Role: constants.ChatMessageRoleUser, Content: "How are you?"},
    },
}

Incorrect Usage Examples:

1. Assistant Message as First Message:
   This will result in an error because the first message must be a user message.

request := &deepseek.ChatCompletionRequest{
    Model: deepseek.DeepSeekReasoner,
    Messages: []deepseek.ChatCompletionMessage{
        {Role: constants.ChatMessageRoleAssistant, Content: "You are a helpful assistant."},
        {Role: constants.ChatMessageRoleUser, Content: "How are you?"},
    },
}

2. System Message as Last Message:
   This will result in an error because the last message must be a user message or an assistant message with prefix mode.

request := &deepseek.ChatCompletionRequest{
    Model: deepseek.DeepSeekReasoner,
    Messages: []deepseek.ChatCompletionMessage{
        {Role: constants.ChatMessageRoleUser, Content: "How are you?"},
        {Role: constants.ChatMessageRoleSystem, Content: "Talk like an anime girl."},
    },
}

Errors

{
  "error": {
    "message": "The first message (except the system message) of deepseek-reasoner must be a user message, but an assistant message detected.",
    "type": "invalid_request_error",
    "param": null,
    "code": "invalid_request_error"
  }
}

{
  "error": {
    "message": "The last message of deepseek-reasoner must be a user message, or an assistant message with prefix mode on (refer to https://api-docs.deepseek.com/guides/chat_prefix_completion).",
    "type": "invalid_request_error",
    "param": null,
    "code": "invalid_request_error"
  }
}