FIX: Cache all flags multisite-safe (#28204)

This fixes an N1 in topics when loading all flags
and also makes the cache multisite-safe, followup
to fb7cc2d3754d23e4c30fa2794eb7ac84ffdc667e

spec/multisite/flags_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe "Custom flags in multisite", type: :multisite do
+  describe "PostACtionType#all_flags" do
+    use_redis_snapshotting
+
+    it "does not share flag definitions between sites" do
+      flag_1 = Flag.create!(name: "test flag 1", position: 99, applies_to: ["Post"])
+
+      test_multisite_connection("second") do
+        flag_2 = Flag.create!(name: "test flag 2", position: 99, applies_to: ["Post"])
+        PostActionType.expire_cache
+        expect(PostActionType.all_flags.last).to eq(
+          flag_2.attributes.except("created_at", "updated_at").transform_keys(&:to_sym),
+        )
+      end
+
+      PostActionType.expire_cache
+      expect(PostActionType.all_flags.last).to eq(
+        flag_1.attributes.except("created_at", "updated_at").transform_keys(&:to_sym),
+      )
+    end
+  end
+end