Add failing test for Site.instance memoization

Site.instance (upstream Hyku) uses first_or_create with no memoization,
so every call is a fresh DB round-trip. Profiling a single /catalog
page load found it called ~650-680 times via the `delegate :account,
..., to: :instance` line alone (see pals-stress-tests repo for the
full profiling writeup). This test asserts the intended fix's
behavior -- currently fails against unmemoized Site.instance.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: maxkadel

spec/models/site_decorator_spec.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+# Site.instance (upstream Hyku, hyrax-webapp/app/models/site.rb) is defined
+# as `first_or_create`, with no memoization -- every call is a fresh
+# database round-trip. Profiling a single /catalog page load found
+# Site.instance called ~650-680 times, almost entirely through the
+# `delegate :account, ..., to: :instance` line, i.e. via the ubiquitous
+# `Site.account` convenience accessor -- not through any one obvious N+1
+# loop. This decorator memoizes it per-request via RequestStore (already a
+# transitive dependency here), the same way `current_account` already is
+# in ApplicationController/HykuHelper.
+RSpec.describe Site, type: :model do
+  before { RequestStore.clear! }
+  after { RequestStore.clear! }
+
+  describe '.instance' do
+    context 'on a specific tenant' do
+      it 'only queries the database once across multiple calls in the same request' do
+        expect(Site).to receive(:first_or_create).once.and_call_original
+
+        3.times { Site.instance }
+      end
+
+      it 'returns the same object across multiple calls' do
+        first = Site.instance
+        second = Site.instance
+
+        expect(first).to equal(second)
+      end
+
+      it 'queries again after RequestStore is cleared (simulating a new request)' do
+        Site.instance
+        RequestStore.clear!
+
+        expect(Site).to receive(:first_or_create).once.and_call_original
+        Site.instance
+      end
+    end
+
+    context 'on global tenant' do
+      before do
+        allow(Account).to receive(:global_tenant?).and_return true
+      end
+
+      it 'is still a NilSite (memoization does not change the existing global-tenant branch)' do
+        expect(Site.instance).to eq(NilSite.instance)
+      end
+    end
+  end
+end