assistant2: Add support for referencing other threads as context (#22092)

This PR adds the ability to reference other threads as context:

<img width="1159" alt="Screenshot 2024-12-16 at 11 29 54 AM"
src="https://github.com/user-attachments/assets/bb8a24ff-56d3-4406-ab8c-6657e65d8c70"
/>

<img width="1159" alt="Screenshot 2024-12-16 at 11 29 35 AM"
src="https://github.com/user-attachments/assets/7a02ebda-a2f5-40e9-9dd4-1bb029cb1c43"
/>


Release Notes:

- N/A

crates/assistant2/src/context.rs

@@ -24,4 +24,5 @@ pub struct Context {
 pub enum ContextKind {
     File,
     FetchedUrl,
+    Thread,
 }