CAPIError 400 invalid message content type <nil>

### Short summary

When using a local model (seems to be any model loaded with Ollama), after one response from the model, any subsequent requests will instantly error with the following error: CAPIError 400 invalid message content type

### Affected version or release

v1.0.25

### Installation context

Installed on Windows 11 desktop, private repo (can provide if needed)

### What happened?

Trying to use a local model served by Ollama, getting the following error after one response from the model:

`CAPIError 400 invalid message content type `

Seems to happen with any model loaded in Ollama that supports tool calling - attempted with `gpt-oss:20b` and `empero-ai/Qwable-9B-Claude-Fable-5-GGUF` (from huggingface) models.

I haven't found any relevant errors in any Github issues, nor any relevant search results. There isn't any workaround except for starting a new session, where I can only make one request before getting the error again.

### Steps to reproduce

1. Add local Ollama provider with any model that supports tool calling
2. In Copilot app, make any request in a new session
3. After first response from the model, make another request
4. See error

### Expected behavior

Subsequent requests should be attempted and respond as normal without errors

### Additional context

_No response_

使用支持 tool-calling 的 Ollama 模型在 Copilot 应用中重现该问题，然后追踪第一次响应之后第二个请求是如何组装的。重点检查变为 nil 的消息内容，并验证在不重启会话的情况下，重复请求能够正常完成。

由索引模型根据 Issue 内容生成。