FEATURE: allow re-scoping chat user search via a plugin (#26361)

This enables the following in Discourse AI

```
 plugin.register_modifier(:chat_allowed_bot_user_ids) do |user_ids, guardian|
  if guardian.user
    mentionables = AiPersona.mentionables(user: guardian.user)
    allowed_bot_ids = mentionables.map { |mentionable| mentionable[:user_id] }
    user_ids.concat(allowed_bot_ids)
  end
  user_ids
end
```

some bots that are id < 0 need to be discoverable in search otherwise people can not talk to them.

---------

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

spec/models/user_spec.rb

@@ -1781,6 +1781,17 @@ RSpec.describe User do
     end
   end
 
+  describe "real users" do
+    it "should find system user if you allow it" do
+      ids =
+        User
+          .real(allowed_bot_user_ids: [Discourse.system_user.id])
+          .where(id: Discourse.system_user.id)
+          .pluck(:id)
+      expect(ids).to eq([Discourse.system_user.id])
+    end
+  end
+
   describe "#purge_unactivated" do
     fab!(:user) { Fabricate(:user, refresh_auto_groups: true) }
     fab!(:unactivated) { Fabricate(:user, active: false) }