chat: support representing file deletions in edit sessions (#289270)

* chat: support representing file deletions in edit sessions

Add support for file deletion operations in chat editing sessions through
new ChatResponseWorkspaceEditPart. This allows agents to represent file-level
operations (deletions, creations, renames) alongside text and notebook edits.

- Adds ChatEditingDeletedFileEntry class to represent deleted files with
  proper diff display and restoration functionality
- Introduces ChatResponseWorkspaceEditPart and IChatWorkspaceEdit interfaces
  for file-level operations at the workspace level
- Implements workspace edit streaming through chat editing sessions with
  proper undo/redo support
- Adds ChatWorkspaceEditContentPart for UI rendering of file operations
- Extends chat session storage to serialize deleted file entries
- Supports both acceptance and rejection of file deletions with proper state
  management and snapshotting

Ref https://github.com/microsoft/vscode/issues/275705

(Commit message generated by Copilot)

* fix test