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/useAutomationValues.js

@@ -121,8 +121,19 @@ export default function useAutomationValues() {
    * @returns {Array} An array of action dropdown values.
    */
   const getActionDropdownValues = type => {
+    let agentsList = agents.value;
+    if (type === 'assign_agent') {
+      agentsList = [
+        {
+          id: 'last_responding_agent',
+          name: t('AUTOMATION.LAST_RESPONDING_AGENT'),
+        },
+        ...agentsList,
+      ];
+    }
+
     return getActionOptions({
-      agents: agents.value,
+      agents: agentsList,
       labels: labels.value,
       teams: teams.value,
       slaPolicies: slaPolicies.value,