Introduce models for Receptive Agents (Part 1)

What does this MR do and why?

This change set introduces the model for Receptive Agents (see &12180).

The commits are suited to be reviewed individually.

This MR is part one of a series of MRs to introduce basic Receptive Agents support in the backend:

1. Introduce models for Receptive Agents (Part 1) (!163142 - merged)
2. Receptive Agents REST API (Part 2) (!163143 - merged)
3. Receptive Agents internal REST API (Part 3) (!163144 - merged)
4. Introduce Cluster Agent URL Configuration Graph... (!163493 - merged)

Refs #463426 (closed) #463427 (closed)

app/models/clusters/agent.rb

@@ -4,6 +4,7 @@ module Clusters
   class Agent < ApplicationRecord
     include FromUnion
     include Gitlab::Utils::StrongMemoize
+    include IgnorableColumns
 
     self.table_name = 'cluster_agents'
 
@@ -38,9 +39,7 @@ class Agent < ApplicationRecord
     scope :with_name, ->(name) { where(name: name) }
     scope :has_vulnerabilities, ->(value = true) { where(has_vulnerabilities: value) }
 
-    # NOTE: we are currently unable to use ignore_column because
-    # it's incompatible with functions in this model.
-    # ignore_column :connection_mode, remove_with: '17.6', remove_after: '2024-11-01'
+    ignore_column :connection_mode, remove_with: '17.6', remove_after: '2024-11-01'
 
     validates :name,
       presence: true,
@@ -94,7 +93,7 @@ def user_access_authorizations
       self.class.from_union(
         user_access_project_authorizations.select('config').limit(1),
         user_access_group_authorizations.select('config').limit(1)
-      ).compact.first
+      ).select('config').compact.first
     end
 
     private