DEV: Add Plugin modifier for reviewable creation (bot posts) (#33161)

I need reviewables to be created for bot users in a private plugin. This
allows me to do that.

lib/post_action_creator.rb

@@ -384,7 +384,17 @@ class PostActionCreator
 
   def create_reviewable(result)
     return unless flagging_post?
-    return if @post.user_id.to_i < 0
+
+    # Return early if the reviewable is being created for a user with a negative user_id.
+    # Plugin apply_modifier can remove this early return for special cases.
+    is_bot_post = @post.user_id.to_i < 0
+    if DiscoursePluginRegistry.apply_modifier(
+         :post_action_creator_block_reviewable_for_bot,
+         is_bot_post,
+         @post,
+       )
+      return
+    end
 
     result.reviewable =
       ReviewableFlaggedPost.needs_review!(

spec/lib/post_action_creator_spec.rb

@@ -196,6 +196,31 @@ RSpec.describe PostActionCreator do
       end
     end
 
+    describe "non-human user being flagged" do
+      fab!(:system_post) { Fabricate(:post, user: Discourse.system_user) }
+
+      it "doesn't create reviewable" do
+        result = PostActionCreator.create(user, system_post, :inappropriate)
+        expect(result.success?).to eq(true)
+
+        expect(result.reviewable).to be_blank
+      end
+
+      it "applies modifier and can allow reviewable creation for non-human users" do
+        plugin = Plugin::Instance.new
+        modifier = :post_action_creator_block_reviewable_for_bot
+        proc = Proc.new { false }
+        DiscoursePluginRegistry.register_modifier(plugin, modifier, &proc)
+
+        result = PostActionCreator.create(user, system_post, :inappropriate)
+        expect(result.success?).to eq(true)
+
+        expect(result.reviewable).to be_present
+      ensure
+        DiscoursePluginRegistry.unregister_modifier(plugin, modifier, &proc)
+      end
+    end
+
     context "with existing reviewable" do
       let!(:reviewable) do
         PostActionCreator.create(