agent_ui: Fix mention and slash command menu not appearing with show_completions_on_input set to false (#44222)

Addresses a regression introduced by
https://github.com/zed-industries/zed/pull/44021 that caused @mentions
and slash commands to stop working if you set
`show_completions_on_input: false` in your settings.

In this case, we should always show these menus, otherwise the features
won't work at all.

Release Notes:

- N/A

crates/agent_ui/src/acp/message_editor.rs

@@ -124,6 +124,7 @@ impl MessageEditor {
             let mut editor = Editor::new(mode, buffer, None, window, cx);
             editor.set_placeholder_text(placeholder, window, cx);
             editor.set_show_indent_guides(false, cx);
+            editor.set_show_completions_on_input(Some(true));
             editor.set_soft_wrap();
             editor.set_use_modal_editing(true);
             editor.set_context_menu_options(ContextMenuOptions {