feat: Introduce last responding agent option to automation assign agent (#12326)

Introduce a `Last Responding Agent` options to assign_agents action in
automations to cover the following use cases.

- Assign conversations to first responding agent : ( automation message
created at , if assignee is nil, assign last responding agent )
- Ensure conversations are not resolved with out an assignee : (
automation conversation resolved at : if assignee is nil, assign last
responding agent )

and potential other cases.

fixes: #1592

app/services/action_service.rb

@@ -43,10 +43,10 @@ class ActionService
   def assign_agent(agent_ids = [])
     return @conversation.update!(assignee_id: nil) if agent_ids[0] == 'nil'
 
+    agent_ids = [last_responding_agent_id] if agent_ids[0] == 'last_responding_agent'
     return unless agent_belongs_to_inbox?(agent_ids)
 
     @agent = @account.users.find_by(id: agent_ids)
-
     return unless @agent.present? && @agent.confirmed?
 
     @conversation.update!(assignee_id: @agent.id)
@@ -95,6 +95,10 @@ class ActionService
 
   private
 
+  def last_responding_agent_id
+    @conversation.messages.outgoing.where(sender_type: 'User', private: false).last&.sender_id
+  end
+
   def agent_belongs_to_inbox?(agent_ids)
     member_ids = @conversation.inbox.members.pluck(:user_id)
     assignable_agent_ids = member_ids + @account.administrators.ids