FIX: Set X-Robots-Tag header to prevent indexing of /safe-mode (#32329)

This change adds the `X-Robots-Tag` header to the `/safe-mode` response, discouraging search engines from including the page in their index.

app/controllers/safe_mode_controller.rb

@@ -8,6 +8,7 @@ class SafeModeController < ApplicationController
   skip_before_action :preload_json, :check_xhr
 
   def index
+    response.headers["X-Robots-Tag"] = "noindex, nofollow"
   end
 
   def enter

spec/requests/safe_mode_controller_spec.rb

@@ -21,6 +21,11 @@ RSpec.describe SafeModeController do
       expect(response.body).not_to include("My Custom Header")
       expect(response.body).not_to include("data-theme-id=\"#{theme.id}\"")
     end
+
+    it "sets the robots header" do
+      get "/safe-mode"
+      expect(response.headers["X-Robots-Tag"]).to eq("noindex, nofollow")
+    end
   end
 
   describe "enter" do