assistant2: Wire up basic @-mention interaction for context (#22197)

This PR adds an initial version of using `@` in the message editor to
add context to the thread.

We don't yet insert any sort of reference to it in the message body
itself.

Release Notes:

- N/A

crates/assistant2/src/context_picker/thread_context_picker.rs

@@ -6,7 +6,7 @@ use picker::{Picker, PickerDelegate};
 use ui::{prelude::*, ListItem};
 
 use crate::context::ContextKind;
-use crate::context_picker::ContextPicker;
+use crate::context_picker::{ConfirmBehavior, ContextPicker};
 use crate::context_store;
 use crate::thread::ThreadId;
 use crate::thread_store::ThreadStore;
@@ -20,10 +20,15 @@ impl ThreadContextPicker {
         thread_store: WeakModel<ThreadStore>,
         context_picker: WeakView<ContextPicker>,
         context_store: WeakModel<context_store::ContextStore>,
+        confirm_behavior: ConfirmBehavior,
         cx: &mut ViewContext<Self>,
     ) -> Self {
-        let delegate =
-            ThreadContextPickerDelegate::new(thread_store, context_picker, context_store);
+        let delegate = ThreadContextPickerDelegate::new(
+            thread_store,
+            context_picker,
+            context_store,
+            confirm_behavior,
+        );
         let picker = cx.new_view(|cx| Picker::uniform_list(delegate, cx));
 
         ThreadContextPicker { picker }
@@ -52,6 +57,7 @@ pub struct ThreadContextPickerDelegate {
     thread_store: WeakModel<ThreadStore>,
     context_picker: WeakView<ContextPicker>,
     context_store: WeakModel<context_store::ContextStore>,
+    confirm_behavior: ConfirmBehavior,
     matches: Vec<ThreadContextEntry>,
     selected_index: usize,
 }
@@ -61,11 +67,13 @@ impl ThreadContextPickerDelegate {
         thread_store: WeakModel<ThreadStore>,
         context_picker: WeakView<ContextPicker>,
         context_store: WeakModel<context_store::ContextStore>,
+        confirm_behavior: ConfirmBehavior,
     ) -> Self {
         ThreadContextPickerDelegate {
             thread_store,
             context_picker,
             context_store,
+            confirm_behavior,
             matches: Vec::new(),
             selected_index: 0,
         }
@@ -180,6 +188,11 @@ impl PickerDelegate for ThreadContextPickerDelegate {
                 context_store.insert_context(ContextKind::Thread, entry.summary.clone(), text);
             })
             .ok();
+
+        match self.confirm_behavior {
+            ConfirmBehavior::KeepOpen => {}
+            ConfirmBehavior::Close => self.dismissed(cx),
+        }
     }
 
     fn dismissed(&mut self, cx: &mut ViewContext<Picker<Self>>) {