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/context_strip.rs

@@ -39,6 +39,7 @@ impl ContextStrip {
     pub fn new(
         context_store: Model<ContextStore>,
         workspace: WeakView<Workspace>,
+        editor: WeakView<Editor>,
         thread_store: Option<WeakModel<ThreadStore>>,
         context_picker_menu_handle: PopoverMenuHandle<ContextPicker>,
         suggest_context_kind: SuggestContextKind,
@@ -49,6 +50,7 @@ impl ContextStrip {
                 workspace.clone(),
                 thread_store.clone(),
                 context_store.downgrade(),
+                editor.clone(),
                 ConfirmBehavior::KeepOpen,
                 cx,
             )