Responses API: AssistantReasoningDeltaEvent and AssistantReasoningEvent triggered with empty content via BYOK providers (Copilot CLI)

### Describe the bug

When using GitHub Copilot CLI (or SDK) with BYOK—either OpenAI or vLLM—for Responses API, both AssistantReasoningDeltaEvent and AssistantReasoningEvent are triggered but with empty content, even if the provider's response includes reasoning content.

### Affected version

1.0.43

### Steps to reproduce the behavior

1. Set up Copilot CLI to use BYOK, OpenAI or vLLM provider (Windows/WSL). Sample .NET code for SDK (works similarly with CLI):

```csharp
await using var session = await client.CreateSessionAsync(new SessionConfig
{
Provider = new ProviderConfig
{
Type = "openai",
BaseUrl = "https://api.openai.com/v1",
ApiKey = "your_api_key",
WireApi = "responses",
},
ReasoningEffort = "high",
Model = "gpt-5.4-2026-03-05",
OnPermissionRequest = PermissionHandler.ApproveAll,
Streaming = true
});
```

2. Trigger completions or calls that should generate reasoning via the Responses API.
3. See that the events are always empty.

### Expected behavior

These events should present the reasoning/explanation content if the provider response includes it. Content field should not be blank if a reasoning field is present.

### Additional context

- Environment: Windows, WSL
- Copilot CLI version: 1.0.43
- Copilot SDK version: 0.3.0
- BYOK provider: OpenAI or vLLM

未指定实现文件或测试。使用为 OpenAI 或 vLLM BYOK 提供商显示的 Responses API 配置重现该问题，然后跟踪 reasoning 事件处理。完成标准是 AssistantReasoningDeltaEvent 和 AssistantReasoningEvent 暴露提供商的 reasoning 内容，而不是空内容。

由索引模型根据 Issue 内容生成。