fix: return model errors instead of raw exception in regenerate_join_code

Returning e.message leaked the 'Validation failed: …' prefix and
inconsistent shape vs. the other actions in this controller, which
return a clean { error: … } payload. Use the model's full_messages
joined to a sentence to keep the API contract uniform.

Author: fspeirs

app/controllers/api/school_classes_controller.rb

@@ -85,8 +85,8 @@ def regenerate_join_code
       @school_class.regenerate_join_code!
       @school_class_with_teachers = @school_class.with_teachers
       render :show, formats: [:json], status: :ok
-    rescue ActiveRecord::RecordInvalid => e
-      render json: { error: e.message }, status: :unprocessable_content
+    rescue ActiveRecord::RecordInvalid
+      render json: { error: @school_class.errors.full_messages.to_sentence }, status: :unprocessable_content
     end
 
     private