assistant2: Add ability to open the active thread as Markdown (#26690)

This PR adds a new `assistant2: open active thread as markdown` action
that opens up the active thread in a Markdown representation:

<img width="1394" alt="Screenshot 2025-03-13 at 12 25 33 PM"
src="https://github.com/user-attachments/assets/363baaaa-c74b-4e93-af36-a3e04a114af0"
/>

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -53,7 +53,8 @@ actions!(
         FocusLeft,
         FocusRight,
         RemoveFocusedContext,
-        AcceptSuggestedContext
+        AcceptSuggestedContext,
+        OpenActiveThreadAsMarkdown
     ]
 );