Fix UrlGenerationError when visiting invalid course slug (#8610)

When visiting /courses/<invalid-slug>, the use_course! callback
tried to redirect to a non-existent :coding_fundamentals action,
causing an ActionController::UrlGenerationError. Redirect to
jiki_url instead, consistent with other redirects in the controller.

Closes #8592

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/controllers/courses_controller.rb

@@ -129,7 +129,7 @@ def redirect_non_members_to_jiki!
   def use_course!
     @bundle = Courses::BundleCodingFrontEnd.instance
     @course = Courses::Course.course_for_slug(params[:id])
-    redirect_to action: :coding_fundamentals unless @course
+    redirect_to jiki_url unless @course
   end
 
   def use_enrollment!