DEV: Prevent detect_text_locale from appearing in menus (#967)

### 🔍 Overview
With the recent changes to allow DiscourseAi in the translator plugin, `detect_text_locale` was needed as a CompletionPrompt. However, it is leaking into composer/post helper menus. This PR ensures we don't not show it in those menus.

lib/ai_helper/assistant.rb

@@ -233,6 +233,8 @@ module DiscourseAi
           %w[post]
         when "illustrate_post"
           %w[composer]
+        when "detect_text_locale"
+          %w[]
         else
           %w[composer post]
         end