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/javascript/dashboard/composables/spec/useAutomation.spec.js

@@ -92,7 +92,9 @@ describe('useAutomation', () => {
         case 'assign_team':
           return teams;
         case 'assign_agent':
-          return agents;
+          return options.addNoneToListFn
+            ? options.addNoneToListFn(options.agents)
+            : options.agents;
         case 'send_email_to_team':
           return teams;
         case 'send_message':
@@ -240,7 +242,11 @@ describe('useAutomation', () => {
 
     expect(getActionDropdownValues('add_label')).toEqual(labels);
     expect(getActionDropdownValues('assign_team')).toEqual(teams);
-    expect(getActionDropdownValues('assign_agent')).toEqual(agents);
+    expect(getActionDropdownValues('assign_agent')).toEqual([
+      { id: 'nil', name: 'AUTOMATION.NONE_OPTION' },
+      { id: 'last_responding_agent', name: 'AUTOMATION.LAST_RESPONDING_AGENT' },
+      ...agents,
+    ]);
     expect(getActionDropdownValues('send_email_to_team')).toEqual(teams);
     expect(getActionDropdownValues('send_message')).toEqual([]);
     expect(getActionDropdownValues('add_sla')).toEqual(slaPolicies);