Clean up streaming codeblocks + edits

[roblourens]

From https://github.com/microsoft/vscode-copilot/issues/12961:

The agent have to stream a codeblock annotated with a URI, then TextEdits are streamed for it async. Today, we expect:

• Markdown codeblock is opened
• codeblockUri response part is streamed
• Codeblock contents are streamed
• TextEdits are streamed and associated with that codeblock, these can come after other content was streamed

This is a bit complicated, eg leads to this weird code in the editfile tool

vscode/src/vs/workbench/contrib/chat/common/tools/editFileTool.ts

Lines 98 to 109 in c659faf

	model.acceptResponseProgress(request, {
	kind: 'markdownContent',
	content: new MarkdownString('\n````\n')
	});
	model.acceptResponseProgress(request, {
	kind: 'codeblockUri',
	uri
	});
	model.acceptResponseProgress(request, {
	kind: 'markdownContent',
	content: new MarkdownString(parameters.code + '\n````\n')
	});

Since either Edits or agent mode know that a file is being edited, we add complexity by going edit -> markdown -> special rendered pill, when the response stream could just include an entry indicating a codeblock with some URI and content. The disconnect between the markdown and the codeblockURI part adds confusion in the code that handles these things.

One idea:

Just have a codeblock response stream part. In the API it looks like

codeblockStart(uri);
codeblockContent(content: string);
codeblockEnd();

In the ChatModel, we merge these calls into a codeblock response part. Rendering is much simpler, we don't have to parse markdown or wait for the codeblockUri to come in. TextEdits also get associated with the codeblock somehow.