copilot: support 'extra_body' for BYOK (top_k, chat_template_kwargs, …

[gpotter2]

Many inference engines have extra 'custom' attributes that are out of the classic responses / chat-completions format. This is commonly referred to as 'extra_body', because that's how it was called in the OpenAI API.

• vLLM has for instance top_k, guided_choice, etc. https://docs.vllm.ai/en/v0.8.3/serving/openai_compatible_server.html
• SGLang has support for a bunch like chat_template_kwargs, for instance when using GLM 5.2 on a custom endpoint: https://docs.sglang.io/cookbook/autoregressive/GLM/GLM-5.2#3-1-reasoning
• deepseek has a special "thinking" parameter in their doc, that they document using extra_body: https://api-docs.deepseek.com/guides/thinking_mode/
• when using QwenCloud, a similar 'enable_thinking' parameter can be provided: https://docs.qwencloud.com/developer-guides/text-generation/thinking
• langchain gives some other examples for LM Studio: https://reference.langchain.com/python/langchain-openai/chat_models/base/BaseChatOpenAI/extra_body

This PR:

• adds 'extraBody' to mimic this functionality. This is a generalization of modelOptions, which currently has a hardcoded list of two allowed values. Arguably this could replace 'modelOptions', but for now I chose to keep this backwards compatible.
• fixes a 'TODO' in 'requestLoggerImpl.ts' to include those parameters in the debug log

Some examples:

• SGLang with GLM 5.2 (I tested this one)

"extraBody": {
	"chat_template_kwargs": {
		"enable_thinking": false
	}
}

• vLLM top_k

"extraBody": {
	"top_k": 50
}

This comes with a side PR with the doc :D microsoft/vscode-docs#10101

fixes #321372

[Copilot]

Pull request overview

Adds BYOK extraBody support for forwarding provider-specific request parameters to OpenAI-compatible endpoints.

Changes:

• Propagates extraBody through BYOK model configuration and metadata.
• Merges extra properties into all supported request-body formats.
• Expands request logging and adds metadata propagation coverage.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
extensions/copilot/src/platform/endpoint/common/endpointProvider.ts	Adds endpoint metadata for extra body properties.
extensions/copilot/src/extension/prompt/vscode-node/requestLoggerImpl.ts	Logs additional request-body options.
extensions/copilot/src/extension/byok/vscode-node/customOAIProvider.ts	Propagates custom OpenAI configuration.
extensions/copilot/src/extension/byok/vscode-node/customEndpointProvider.ts	Propagates custom endpoint configuration.
extensions/copilot/src/extension/byok/vscode-node/azureProvider.ts	Propagates Azure BYOK configuration.
extensions/copilot/src/extension/byok/node/openAIEndpoint.ts	Applies extra properties to outgoing bodies.
extensions/copilot/src/extension/byok/common/test/byokProvider.spec.ts	Tests metadata propagation.
extensions/copilot/src/extension/byok/common/byokProvider.ts	Adds shared BYOK capability propagation.