Wire up @mentioned files in assistant2 (#23389)

`@mention`ed files in assistant2 now get replaced by the full path of
the file in what gets sent to the model, while rendering visually as
just the filename (in a crease, so they can only be selected/deleted as
a whole unit, not character by character).


https://github.com/user-attachments/assets/a5867a93-d656-4a17-aced-58424c6e8cf6

Release Notes:

- N/A

---------

Co-authored-by: João Marcos <joao@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>

crates/assistant2/src/message_editor.rs

@@ -64,6 +64,7 @@ impl MessageEditor {
                 workspace.clone(),
                 Some(thread_store.clone()),
                 context_store.downgrade(),
+                editor.downgrade(),
                 ConfirmBehavior::Close,
                 cx,
             )
@@ -73,6 +74,7 @@ impl MessageEditor {
             ContextStrip::new(
                 context_store.clone(),
                 workspace.clone(),
+                editor.downgrade(),
                 Some(thread_store.clone()),
                 context_picker_menu_handle.clone(),
                 SuggestContextKind::File,