FIX: Category hashtags weren't always found for sub-sub-categories (#20156)

The algorithm failed to find the correct category by slug when there are multiple sub-sub-categories with the same child-category name and the first child doesn't  have the correct grandchild.

So, searching for "child / grandchild" worked in the following case, it found (3):

- (1) parent 1
  - (2) child
    - (3) grandchild
- (4) parent 2
  - (5) child
    - (6) grandchild

But it failed to find the grandchild in the following case:

- (1) parent 1
  - (2) child
- (4) parent 2
  - (5) child
    - (6) grandchild

And this also fixes a flaky spec by forcing categories to always order by by `parent_category_id` and `id`.
This makes it possible to partly revert 60990aab55

spec/requests/hashtags_controller_spec.rb

@@ -275,6 +275,8 @@ RSpec.describe HashtagsController do
 
             qux = Fabricate(:category_with_definition, slug: "qux")
             quxbar = Fabricate(:category_with_definition, slug: "bar", parent_category_id: qux.id)
+            quxbarbaz =
+              Fabricate(:category_with_definition, slug: "baz", parent_category_id: quxbar.id)
 
             invalid_slugs = [":"]
             child_slugs = %w[bar baz]

spec/services/category_hashtag_data_source_spec.rb

@@ -40,6 +40,47 @@ RSpec.describe CategoryHashtagDataSource do
       group.add(user)
       expect(described_class.lookup(Guardian.new(user), ["secret"]).first).not_to eq(nil)
     end
+
+    context "with sub-sub-categories" do
+      before { SiteSetting.max_category_nesting = 3 }
+
+      it "returns the first matching grandchild category (ordered by IDs) when there are multiple categories with the same slug" do
+        parent1 = Fabricate(:category, slug: "parent1")
+        parent2 = Fabricate(:category, slug: "parent2")
+
+        parent1_child = Fabricate(:category, slug: "child", parent_category_id: parent1.id)
+        parent1_child_grandchild =
+          Fabricate(:category, slug: "grandchild", parent_category_id: parent1_child.id)
+
+        parent2_child = Fabricate(:category, slug: "child", parent_category_id: parent2.id)
+        parent2_child_grandchild =
+          Fabricate(:category, slug: "grandchild", parent_category_id: parent2_child.id)
+
+        result = described_class.lookup(guardian, ["child:grandchild"])
+        expect(result.map(&:relative_url)).to eq([parent1_child_grandchild.url])
+
+        parent1_child.destroy
+        parent1_child = Fabricate(:category, slug: "child", parent_category_id: parent1.id)
+
+        result = described_class.lookup(guardian, ["child:grandchild"])
+        expect(result.map(&:relative_url)).to eq([parent2_child_grandchild.url])
+      end
+
+      it "returns the correct grandchild category when there are multiple children with the same slug and only one of them has the correct grandchild" do
+        parent1 = Fabricate(:category, slug: "parent1")
+        parent1_child = Fabricate(:category, slug: "child", parent_category_id: parent1.id)
+        parent1_child_grandchild =
+          Fabricate(:category, slug: "another-grandchild", parent_category_id: parent1_child.id)
+
+        parent2 = Fabricate(:category, slug: "parent2")
+        parent2_child = Fabricate(:category, slug: "child", parent_category_id: parent2.id)
+        parent2_child_grandchild =
+          Fabricate(:category, slug: "grandchild", parent_category_id: parent2_child.id)
+
+        result = described_class.lookup(guardian, ["child:grandchild"])
+        expect(result.map(&:relative_url)).to eq([parent2_child_grandchild.url])
+      end
+    end
   end
 
   describe "#search" do