Tweak word completions more (#37697)

Follow-up of https://github.com/zed-industries/zed/pull/37352
Closes https://github.com/zed-industries/zed/issues/37132

* disabled word completions in the agent panel's editor
* if not disabled, allow to trigger word completions with an action even
if the completions threshold is not reached

Release Notes:

- Fixed word completions appearing in the agent panel's editor and not
appearing when triggered with the action before the completion threshold
is reached

crates/agent_ui/src/message_editor.rs

@@ -125,6 +125,7 @@ pub(crate) fn create_editor(
             cx,
         );
         editor.set_placeholder_text("Message the agent – @ to include context", cx);
+        editor.disable_word_completions();
         editor.set_show_indent_guides(false, cx);
         editor.set_soft_wrap();
         editor.set_use_modal_editing(true);