fix(search): guard reindex_for_search when searchkick is not loaded

searchkick (which defines #reindex) is only mixed into Message at class-load time
when advanced_search_allowed? (enterprise + OPENSEARCH_URL) is true at boot. A spec
that stubs advanced_search_allowed? true without a loaded index makes should_index?
true and fires the reindex callback, raising NoMethodError. Guard on respond_to?
so it no-ops when no index is available (real deployments without search index
already have should_index? false, so behavior is unchanged).

app/models/message.rb

@@ -452,6 +452,10 @@ class Message < ApplicationRecord
   end
 
   def reindex_for_search
+    # searchkick (which defines #reindex) is only mixed in when advanced_search_allowed? is true
+    # at boot; guard so a should_index? true without a loaded index can't raise NoMethodError.
+    return unless respond_to?(:reindex)
+
     reindex(mode: :async)
   end
 end