FEATURE: Include advanced search option to include unlisted topics in the results (#27958)

---------

Co-authored-by: Régis Hanol <regis@hanol.fr>

spec/lib/search_spec.rb

@@ -2541,6 +2541,23 @@ RSpec.describe Search do
     end
   end
 
+  describe "include:invisible / include:unlisted" do
+    it "allows including invisible topics in the results" do
+      topic = Fabricate(:topic, title: "I am testing a search", visible: false)
+      post = Fabricate(:post, topic: topic, raw: "this is the first post", post_number: 1)
+      _post2 = Fabricate(:post, topic: topic, raw: "this is the second post", post_number: 2)
+
+      results = Search.execute("testing include:invisible")
+      expect(results.posts.map(&:id)).to eq([post.id])
+
+      results = Search.execute("testing include:unlisted")
+      expect(results.posts.map(&:id)).to eq([post.id])
+
+      results = Search.execute("testing")
+      expect(results.posts).to eq([])
+    end
+  end
+
   describe "ignore_diacritics" do
     before { SiteSetting.search_ignore_accents = true }
     let!(:post1) { Fabricate(:post, raw: "สวัสดี Rágis hello") }