DEV: Change HashtagAutocompleteService to use DiscoursePluginRegistry (#19491)

Follow up to a review in #18937, this commit changes the HashtagAutocompleteService to no longer use class variables to register hashtag data sources or types in context priority order. This is to address multisite concerns, where one site could e.g. have chat disabled and another might not. The filtered plugin registers I added will not be included if the plugin is disabled.

spec/support/fake_bookmark_hashtag_data_source.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+class FakeBookmarkHashtagDataSource
+  def self.icon
+    "bookmark"
+  end
+
+  def self.type
+    "bookmark"
+  end
+
+  def self.lookup(guardian_scoped, slugs)
+    guardian_scoped
+      .user
+      .bookmarks
+      .where("LOWER(name) IN (:slugs)", slugs: slugs)
+      .map do |bm|
+        HashtagAutocompleteService::HashtagItem.new.tap do |item|
+          item.text = bm.name
+          item.slug = bm.name.gsub(" ", "-")
+          item.icon = icon
+        end
+      end
+  end
+
+  def self.search(
+    guardian_scoped,
+    term,
+    limit,
+    condition = HashtagAutocompleteService.search_conditions[:starts_with]
+  )
+    query = guardian_scoped.user.bookmarks
+
+    if condition == HashtagAutocompleteService.search_conditions[:starts_with]
+      query = query.where("name ILIKE ?", "#{term}%")
+    else
+      query = query.where("name ILIKE ?", "%#{term}%")
+    end
+
+    query
+      .limit(limit)
+      .map do |bm|
+        HashtagAutocompleteService::HashtagItem.new.tap do |item|
+          item.text = bm.name
+          item.slug = bm.name.gsub(" ", "-")
+          item.icon = icon
+        end
+      end
+  end
+
+  def self.search_sort(search_results, _)
+    search_results.sort_by { |item| item.text.downcase }
+  end
+end