Fix some issues with branch buffers (zed-industries#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
         });

crates/assistant/src/prompt_library.rs

@@ -521,9 +521,9 @@ impl PromptLibrary {
                             editor.set_show_indent_guides(false, cx);
                             editor.set_use_modal_editing(false);
                             editor.set_current_line_highlight(Some(CurrentLineHighlight::None));
-                            editor.set_completion_provider(Box::new(
+                            editor.set_completion_provider(Some(Box::new(
                                 SlashCommandCompletionProvider::new(None, None),
-                            ));
+                            )));
                             if focus {
                                 editor.focus(cx);
                             }

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -111,7 +111,7 @@ impl MessageEditor {
             editor.set_show_gutter(false, cx);
             editor.set_show_wrap_guides(false, cx);
             editor.set_show_indent_guides(false, cx);
-            editor.set_completion_provider(Box::new(MessageEditorCompletionProvider(this)));
+            editor.set_completion_provider(Some(Box::new(MessageEditorCompletionProvider(this))));
             editor.set_auto_replace_emoji_shortcode(
                 MessageEditorSettings::get_global(cx)
                     .auto_replace_emoji_shortcode