add diffs to thinking headers

[justschen]

adds diffs from code blocks into the header for better edit visibility

[Copilot]

Pull request overview

This PR enhances the chat “thinking/working” UI by surfacing aggregated edit diff stats (+/-) from edit code-block pills directly in the thinking header, improving visibility of edit impact while the list is collapsed.

Changes:

• Plumbs an aggregated diff-change event from ChatMarkdownContentPart up into ChatThinkingContentPart.
• Renders the aggregated +added/-removed counts in the thinking header and styles them via CSS.
• Adds diff-change emission from edit code-block pills and aggregates those stats per markdown part.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/vs/workbench/contrib/chat/browser/widget/chatListRenderer.ts	Passes markdown-part diff-change event into thinking-part item appends.
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/media/chatThinkingContent.css	Adds styling for the new diff stats segment in the thinking header.
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatThinkingContentPart.ts	Aggregates diff stats per appended markdown part and renders totals in the finalized title.
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatMarkdownContentPart.ts	Emits aggregated diff stats from edit pills and wires pill diff updates into the aggregation.