Fix some issues with branch buffers (#18945)

* `Open Excerpts` command always opens the locations in the base buffer
* LSP features like document-highlights, go-to-def, and inlay hints work
correctly in branch buffers
* Other LSP features like completions, code actions, and rename are
disabled in branch buffers

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -1560,7 +1560,7 @@ impl ContextEditor {
             editor.set_show_runnables(false, cx);
             editor.set_show_wrap_guides(false, cx);
             editor.set_show_indent_guides(false, cx);
-            editor.set_completion_provider(Box::new(completion_provider));
+            editor.set_completion_provider(Some(Box::new(completion_provider)));
             editor.set_collaboration_hub(Box::new(project.clone()));
             editor
         });