json: cannot unmarshal object into Go struct field

# Environment

codegpt version: 0.14.2 (commit: fd5758c)
Model: [Gemma 2](https://huggingface.co/bartowski/gemma-2-2b-it-GGUF/resolve/main/gemma-2-2b-it-Q6_K.gguf) (via local KoboldCpp 1.72 instance)

I’ve configured codegpt to use a local KoboldCpp instance:

```bash
codegpt config set openai.base_url http://localhost:5001/v1
codegpt config set openai.timeout 1m
```

# Issue Description

The configured koboldcpp endpoint is designed to be compatible with the OpenAI API format, see https://github.com/LostRuins/koboldcpp#Notes. Given this API compatibility, `codegpt review` operates without throwing errors (although the output quality depends on the specific model used). However, `codegpt commit` encounters a persistent issue, resulting in the following error:

```bash
❯ codegpt commit
Summarize the commit message use gpt-3.5-turbo model
We are trying to summarize a git diff
PromptTokens: 100, CompletionTokens: 100, TotalTokens: 200
We are trying to summarize a title for pull request
PromptTokens: 100, CompletionTokens: 100, TotalTokens: 200
We are trying to get conventional commit prefix (Tools)
Error: json: cannot unmarshal object into Go struct field ChatCompletionChoice.choices.message of type string
```

# Expected Behavior

The command should process the commit message successfully, even if the model outputs unusual characters.