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_

Start by locating the BYOK code that assembles the LLM request body and inspect how environment variables are read. Add support for parsing COPILOT_EXTRA_REQUEST_PARAMS as JSON and appending its values to the request body, then verify requests with and without the variable.

Written by the indexing model from the issue text.