FIX: Update category tag stats with new or deleted (#21531)

The old method updated only existing records, without considering that
new tags might have been created or some tags might not exist anymore.
This was usually not a problem because the stats were also updated by
other code paths.

However, the ensure consistency job should be more solid and help when
other code paths fail or after importing data.

Also, update category tag stats too should happen when updating other
category stats as well.

spec/models/category_tag_stat_spec.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+describe CategoryTagStat do
+  fab!(:category) { Fabricate(:category) }
+  fab!(:tag) { Fabricate(:tag) }
+  fab!(:topic) { Fabricate(:topic, category: category, tags: [tag]) }
+
+  describe "#update_topic_counts" do
+    it "creates new records" do
+      CategoryTagStat.destroy_all
+
+      expect { CategoryTagStat.update_topic_counts }.to change { CategoryTagStat.count }.by(1)
+      category_tag_stat = CategoryTagStat.last
+      expect(category_tag_stat.category_id).to eq(category.id)
+      expect(category_tag_stat.tag_id).to eq(tag.id)
+      expect(category_tag_stat.topic_count).to eq(1)
+    end
+
+    it "updates existing records" do
+      CategoryTagStat.last.update(topic_count: 10)
+
+      expect { CategoryTagStat.update_topic_counts }.not_to change { CategoryTagStat.count }
+      category_tag_stat = CategoryTagStat.last
+      expect(category_tag_stat.category_id).to eq(category.id)
+      expect(category_tag_stat.tag_id).to eq(tag.id)
+      expect(category_tag_stat.topic_count).to eq(1)
+    end
+
+    it "deletes old records" do
+      CategoryTagStat.last.update(tag_id: Fabricate(:tag).id)
+
+      expect { CategoryTagStat.update_topic_counts }.not_to change { CategoryTagStat.count }
+      category_tag_stat = CategoryTagStat.last
+      expect(category_tag_stat.category_id).to eq(category.id)
+      expect(category_tag_stat.tag_id).to eq(tag.id)
+      expect(category_tag_stat.topic_count).to eq(1)
+    end
+  end
+end