DEV: Improve distributed cache multisite specs (#30662)

Distributed cache when namespace is false is not multisite safe as
values are shared between sites. Distributed cache with namespace option
(default) is multisite safe.

Improved specs to cover both cases.

spec/multisite/distributed_cache_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe "Multisite SiteSettings", type: :multisite do
   context "without namespace" do
     let(:cache1) { cache("test", namespace: false) }
 
-    it "does not leak state across multisite" do
+    it "does share a global state between multisite" do
       cache1["default"] = true
 
       expect(cache1.hash).to eq("default" => true)
@@ -23,9 +23,35 @@ RSpec.describe "Multisite SiteSettings", type: :multisite do
         expect(message.data[:op]).to eq(:set)
         expect(message.data[:key]).to eq("second")
         expect(message.data[:value]).to eq(true)
+        expect(cache1.hash).to eq("default" => true, "second" => true)
       end
 
       expect(cache1.hash).to eq("default" => true, "second" => true)
     end
   end
+
+  context "with namespace" do
+    let(:cache1) { cache("test", namespace: true) }
+
+    it "does not share a state between multisite" do
+      cache1["default"] = true
+
+      expect(cache1.hash).to eq("default" => true)
+
+      test_multisite_connection("second") do
+        message =
+          MessageBus
+            .track_publish(DistributedCache::Manager::CHANNEL_NAME) { cache1["second"] = true }
+            .first
+
+        expect(message.data[:hash_key]).to eq("test")
+        expect(message.data[:op]).to eq(:set)
+        expect(message.data[:key]).to eq("second")
+        expect(message.data[:value]).to eq(true)
+        expect(cache1.hash).to eq("second" => true)
+      end
+
+      expect(cache1.hash).to eq("default" => true)
+    end
+  end
 end