support extra request params for byok model

### Describe the feature or problem you'd like to solve

some llm providers might require additional nonstandard request parameters for certain features or tweaks

### Proposed solution

env var `COPILOT_EXTRA_REQUEST_PARAMS='{"abc": "def"}'`
so `{"abc": "def"}` will be appended to the llm request body
similar features seems also exist in various other agent frameworks

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先定位组装 LLM 请求体的 BYOK 代码，并检查环境变量的读取方式。添加将 COPILOT_EXTRA_REQUEST_PARAMS 解析为 JSON 并将其值追加到请求体的支持，然后验证有该变量和没有该变量时的请求。

由索引模型根据 Issue 内容生成。