assistant2: Don't suggest thread context for inline assist without a ThreadStore (#23506)

This PR makes it so we don't suggest threads as context in the inline
assist when we don't have a `ThreadStore` to pull from.

Release Notes:

- N/A

crates/assistant2/src/context_picker.rs

@@ -89,7 +89,7 @@ impl ContextPicker {
                 ContextKind::Directory,
                 ContextKind::FetchedUrl,
             ];
-            if self.thread_store.is_some() {
+            if self.allow_threads() {
                 context_kinds.push(ContextKind::Thread);
             }
 
@@ -132,6 +132,11 @@ impl ContextPicker {
         menu
     }
 
+    /// Whether threads are allowed as context.
+    pub fn allow_threads(&self) -> bool {
+        self.thread_store.is_some()
+    }
+
     fn select_kind(&mut self, kind: ContextKind, cx: &mut ViewContext<Self>) {
         let context_picker = cx.view().downgrade();