agent_ui: Fix selection mentions from "Add to Agent Thread" not opening files (#61621)

# Objective

Fixes #59333.

Selection mentions created with "Add to Agent Thread" could not be
clicked to open the referenced file, while selection mentions created by
pasting were navigable.

## Solution

The ready selection-mention crease did not receive its `MentionUri` or
`Workspace`, so `MentionCrease` could not install its click handler.
Pass the workspace through the action-completion callback and provide
both values to the shared crease renderer, enabling its existing
navigation behavior.

## Testing

I tested these changes locally.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

Before:


https://github.com/user-attachments/assets/8dd4a2d0-4b22-4fb3-8777-cf97b6436b91

After:


https://github.com/user-attachments/assets/cff1cdb3-804a-45d8-a9c5-fdd8b35b6f8f


---

Release Notes:

- Fixed selection mentions added with "Add to Agent Thread" not opening
their referenced files.

crates/agent_ui/src/completion_provider.rs

@@ -528,7 +528,14 @@ impl<T: PromptCompletionProviderDelegate> PromptCompletionProvider<T> {
                     AgentContextSource::from_active(workspace, cx)?
                         .read_selection(workspace, false, cx)
                 });
-                Self::completion_for_action(action, source_range, editor, mention_set, selection)
+                Self::completion_for_action(
+                    action,
+                    source_range,
+                    editor,
+                    mention_set,
+                    workspace.downgrade(),
+                    selection,
+                )
             }
         }
     }
@@ -815,6 +822,7 @@ impl<T: PromptCompletionProviderDelegate> PromptCompletionProvider<T> {
         source_range: Range<Anchor>,
         editor: WeakEntity<Editor>,
         mention_set: WeakEntity<MentionSet>,
+        workspace: WeakEntity<Workspace>,
         selection: Option<AgentContextSelection>,
     ) -> Option<Completion> {
         let (new_text, on_action) = match action {
@@ -824,6 +832,7 @@ impl<T: PromptCompletionProviderDelegate> PromptCompletionProvider<T> {
                         source_range.clone(),
                         editor,
                         mention_set,
+                        workspace,
                         editor_selections,
                     )
                 }
@@ -2702,6 +2711,7 @@ fn completion_text_for_editor_selections(
     source_range: Range<Anchor>,
     editor: WeakEntity<Editor>,
     mention_set: WeakEntity<MentionSet>,
+    workspace: WeakEntity<Workspace>,
     editor_selections: Vec<(Entity<Buffer>, Range<text::Anchor>)>,
 ) -> (String, ConfirmCallback) {
     const EDITOR_PLACEHOLDER: &str = "selection ";
@@ -2725,6 +2735,7 @@ fn completion_text_for_editor_selections(
             let editor = editor.clone();
             let selections = selections.clone();
             let mention_set = mention_set.clone();
+            let workspace = workspace.clone();
             let source_range = source_range.clone();
             window.defer(cx, move |window, cx| {
                 if let Some(editor) = editor.upgrade()
@@ -2736,6 +2747,7 @@ fn completion_text_for_editor_selections(
                                 source_range.clone(),
                                 selections,
                                 editor.clone(),
+                                workspace,
                                 window,
                                 cx,
                             )
@@ -2794,6 +2806,8 @@ fn completion_text_for_terminal_selections(
                         mention_uri.name().into(),
                         mention_uri.icon_path(cx),
                         None,
+                        None,
+                        None,
                         range,
                         editor.downgrade(),
                     );

crates/agent_ui/src/inline_prompt_editor.rs

@@ -1615,6 +1615,8 @@ fn insert_message_creases(
                 crease.label.clone(),
                 crease.icon_path.clone(),
                 None,
+                None,
+                None,
                 start..end,
                 cx.weak_entity(),
             )

crates/agent_ui/src/mention_set.rs

@@ -520,6 +520,7 @@ impl MentionSet {
         source_range: Range<text::Anchor>,
         selections: Vec<(Entity<Buffer>, Range<text::Anchor>, Range<usize>)>,
         editor: Entity<Editor>,
+        workspace: WeakEntity<Workspace>,
         window: &mut Window,
         cx: &mut Context<Self>,
     ) {
@@ -559,6 +560,8 @@ impl MentionSet {
                 selection_name(abs_path.as_deref(), &line_range).into(),
                 uri.icon_path(cx),
                 uri.tooltip_text(),
+                Some(uri.clone()),
+                Some(workspace.clone()),
                 range,
                 editor.downgrade(),
             );
@@ -1143,11 +1146,20 @@ pub(crate) fn crease_for_mention(
     label: SharedString,
     icon_path: SharedString,
     tooltip: Option<SharedString>,
+    mention_uri: Option<MentionUri>,
+    workspace: Option<WeakEntity<Workspace>>,
     range: Range<Anchor>,
     editor_entity: WeakEntity<Editor>,
 ) -> Crease<Anchor> {
     let placeholder = FoldPlaceholder {
-        render: render_fold_icon_button(icon_path.clone(), label.clone(), tooltip, editor_entity),
+        render: render_fold_icon_button(
+            icon_path.clone(),
+            label.clone(),
+            tooltip,
+            mention_uri,
+            workspace,
+            editor_entity,
+        ),
         merge_adjacent: false,
         ..Default::default()
     };
@@ -1162,6 +1174,8 @@ fn render_fold_icon_button(
     icon_path: SharedString,
     label: SharedString,
     tooltip: Option<SharedString>,
+    mention_uri: Option<MentionUri>,
+    workspace: Option<WeakEntity<Workspace>>,
     editor: WeakEntity<Editor>,
 ) -> Arc<dyn Send + Sync + Fn(FoldId, Range<Anchor>, &mut App) -> AnyElement> {
     Arc::new({
@@ -1171,6 +1185,8 @@ fn render_fold_icon_button(
                 .unwrap_or_default();
 
             MentionCrease::new(fold_id, icon_path.clone(), label.clone())
+                .mention_uri(mention_uri.clone())
+                .workspace(workspace.clone())
                 .is_toggled(is_in_text_selection)
                 .when_some(tooltip.clone(), |this, tooltip_text| {
                     this.tooltip(tooltip_text)

crates/agent_ui/src/message_editor.rs

@@ -1599,6 +1599,7 @@ impl MessageEditor {
                 anchor..anchor,
                 self.editor.downgrade(),
                 self.mention_set.downgrade(),
+                self.workspace.clone(),
                 Some(selection),
             )
         else {