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_picker.rs

@@ -43,6 +43,7 @@ enum ContextPickerMode {
 pub(super) struct ContextPicker {
     mode: ContextPickerMode,
     workspace: WeakView<Workspace>,
+    editor: WeakView<Editor>,
     context_store: WeakModel<ContextStore>,
     thread_store: Option<WeakModel<ThreadStore>>,
     confirm_behavior: ConfirmBehavior,
@@ -53,6 +54,7 @@ impl ContextPicker {
         workspace: WeakView<Workspace>,
         thread_store: Option<WeakModel<ThreadStore>>,
         context_store: WeakModel<ContextStore>,
+        editor: WeakView<Editor>,
         confirm_behavior: ConfirmBehavior,
         cx: &mut ViewContext<Self>,
     ) -> Self {
@@ -61,6 +63,7 @@ impl ContextPicker {
             workspace,
             context_store,
             thread_store,
+            editor,
             confirm_behavior,
         }
     }
@@ -131,6 +134,7 @@ impl ContextPicker {
                     FileContextPicker::new(
                         context_picker.clone(),
                         self.workspace.clone(),
+                        self.editor.clone(),
                         self.context_store.clone(),
                         self.confirm_behavior,
                         cx,