FIX: if mandatory parent tag is missing, add it

Previous behaviour was to silently remove tags that
belonged to a group with a parent tag that was missing.

The "required parent tag" feature is meant to guide people
to use the correct tags and show scoped results in the tag
input field, and to help create topic lists of related
tags. It isn't meant to be a strict requirement in the
composer that should trigger errors or restrictions.

spec/components/discourse_tagging_spec.rb

@@ -158,6 +158,39 @@ describe DiscourseTagging do
         expect(topic.reload.tags).to eq([hidden_tag])
       end
     end
+
+    context 'tag group with parent tag' do
+      let(:topic) { Fabricate(:topic, user: user) }
+      let(:post) { Fabricate(:post, user: user, topic: topic, post_number: 1) }
+
+      before do
+        tag_group = Fabricate(:tag_group, parent_tag_id: tag1.id)
+        tag_group.tags = [tag3]
+      end
+
+      it "can tag with parent" do
+        valid = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(user), [tag1.name])
+        expect(valid).to eq(true)
+        expect(topic.reload.tags.map(&:name)).to eq([tag1.name])
+      end
+
+      it "can tag with parent and a tag" do
+        valid = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(user), [tag1.name, tag3.name])
+        expect(valid).to eq(true)
+        expect(topic.reload.tags.map(&:name)).to contain_exactly(*[tag1, tag3].map(&:name))
+      end
+
+      it "adds all parent tags that are missing" do
+        parent_tag = Fabricate(:tag, name: 'parent')
+        tag_group2 = Fabricate(:tag_group, parent_tag_id: parent_tag.id)
+        tag_group2.tags = [tag2]
+        valid = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(user), [tag3.name, tag2.name])
+        expect(valid).to eq(true)
+        expect(topic.reload.tags.map(&:name)).to contain_exactly(
+          *[tag1, tag2, tag3, parent_tag].map(&:name)
+        )
+      end
+    end
   end
 
   describe '#tags_for_saving' do