Add coverage for `api/v1/domain_blocks/preview` endpoint (#32303)

spec/requests/api/v1/domain_blocks/previews_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Domain Blocks Previews API' do
+  let(:user)    { Fabricate(:user) }
+  let(:token)   { Fabricate(:accessible_access_token, resource_owner_id: user.id, scopes: scopes) }
+  let(:scopes)  { 'write:blocks' }
+  let(:headers) { { 'Authorization' => "Bearer #{token.token}" } }
+  let(:account) { Fabricate(:account, user: user) }
+
+  describe 'GET /api/v1/domain_blocks/preview' do
+    subject { get '/api/v1/domain_blocks/preview', params: { domain: domain }, headers: headers }
+
+    let(:domain) { 'host.example' }
+
+    before do
+      Fabricate :follow, account: account, target_account: Fabricate(:account, domain: domain)
+      Fabricate :follow, target_account: account, account: Fabricate(:account, domain: domain)
+    end
+
+    it_behaves_like 'forbidden for wrong scope', 'write:statuses'
+
+    it 'returns http success and follower counts' do
+      subject
+
+      expect(response)
+        .to have_http_status(200)
+      expect(response.content_type)
+        .to start_with('application/json')
+      expect(response.parsed_body)
+        .to include(followers_count: 1)
+        .and include(following_count: 1)
+    end
+  end
+end