Handle all ChatResponsePart types in Assistant

[timtmok]

There are other chat response types that we might encounter that aren't currently handled. This came up when using the Shiny chat participant and the chat history contained a ChatResponseCommandButtonPart.

It might be worth adding something to the Echo test provider to respond with parts like this.

positron/src/vscode-dts/vscode.d.ts

Lines 19996 to 20000 in a15f742

	/**
	* Represents the different chat response types.
	*/
	export type ChatResponsePart = ChatResponseMarkdownPart | ChatResponseFileTreePart | ChatResponseAnchorPart
	| ChatResponseProgressPart | ChatResponseReferencePart | ChatResponseCommandButtonPart;

Originally posted by @sharon-wang in #10979 (comment)