Add basic coverage for `ProcessHashtagsService` class (#29320)

spec/services/process_hashtags_service_spec.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ProcessHashtagsService do
+  describe '#call' do
+    let(:status) { Fabricate(:status, visibility: :public, text: 'With tags #one #two') }
+
+    it 'applies the tags from the status text' do
+      expect { subject.call(status) }
+        .to change(Tag, :count).by(2)
+      expect(status.reload.tags.map(&:name))
+        .to contain_exactly('one', 'two')
+    end
+  end
+end