FIX: Push category hashtag slug match to top (#19174)

When searching for categories it is possible for
a child category to have a slug that matches the term
exactly, but will not be found by .lookup since we
don't return these categories unless the ref matches
parent:child.

Introduces a search_sort method to each hashtag data
source so they can provide their custom sort logic of
results, in category's case putting all matching slugs
to the top regardless of parent/child relationship
then sorting by text.

spec/services/hashtag_autocomplete_service_spec.rb

@@ -43,6 +43,10 @@ RSpec.describe HashtagAutocompleteService do
           end
         end
     end
+
+    def self.search_sort(search_results, _)
+      search_results.sort_by { |item| item.text.downcase }
+    end
   end
 
   describe ".contexts_with_ordered_types" do
@@ -166,6 +170,37 @@ RSpec.describe HashtagAutocompleteService do
       )
     end
 
+    it "orders categories by exact match on slug (ignoring parent/child distinction) then name, and then name for everything else" do
+      category2 = Fabricate(:category, name: "Book Library", slug: "book-library")
+      Fabricate(:category, name: "Horror", slug: "book", parent_category: category2)
+      Fabricate(:category, name: "Romance", slug: "romance-books")
+      Fabricate(:category, name: "Abstract Philosophy", slug: "abstract-philosophy-books")
+      category6 = Fabricate(:category, name: "Book Reviews", slug: "book-reviews")
+      Fabricate(:category, name: "Good Books", slug: "book", parent_category: category6)
+      expect(subject.search("book", %w[category]).map(&:ref)).to eq(
+        %w[
+          book-reviews:book
+          book-library:book
+          abstract-philosophy-books
+          book-club
+          book-library
+          book-reviews
+          romance-books
+        ],
+      )
+      expect(subject.search("book", %w[category]).map(&:text)).to eq(
+        [
+          "Good Books",
+          "Horror",
+          "Abstract Philosophy",
+          "Book Club",
+          "Book Library",
+          "Book Reviews",
+          "Romance",
+        ],
+      )
+    end
+
     context "when multiple tags and categories are returned" do
       fab!(:category2) { Fabricate(:category, name: "Book Zone", slug: "book-zone") }
       fab!(:category3) { Fabricate(:category, name: "Book Dome", slug: "book-dome") }