FIX: No small action created when a non-author removes itself from a PM (#20502)

Fixes a small issue where allowed user removes themselves from a private message before the post activity (small action) is created.

I also added some test coverage to prevent regression.

/t/92811

spec/models/topic_spec.rb

@@ -3081,6 +3081,17 @@ RSpec.describe Topic do
 
   describe "#remove_allowed_user" do
     fab!(:topic) { Fabricate(:topic) }
+    fab!(:private_topic) do
+      Fabricate(
+        :private_message_topic,
+        title: "Private message",
+        user: admin,
+        topic_allowed_users: [
+          Fabricate.build(:topic_allowed_user, user: admin),
+          Fabricate.build(:topic_allowed_user, user: user1),
+        ],
+      )
+    end
 
     describe "removing oneself" do
       it "should remove onself" do
@@ -3095,6 +3106,12 @@ RSpec.describe Topic do
         expect(post.post_type).to eq(Post.types[:small_action])
         expect(post.action_code).to eq("user_left")
       end
+
+      it "should show a small action when user removes themselves from pm" do
+        expect do private_topic.remove_allowed_user(user1, user1) end.to change {
+          private_topic.posts.where(action_code: "user_left").count
+        }.by(1)
+      end
     end
   end