feat: Add private note action to automations (#11926)

## Summary
- allow AutomationRule to accept `add_private_note` action
- support `add_private_note` in automation action service
- expose private note action in frontend constants and i18n
- test new automation rule action

## Testing
- `pnpm eslint
app/javascript/dashboard/routes/dashboard/settings/automation/constants.js`
- `bundle exec rubocop app/services/automation_rules/action_service.rb
app/models/automation_rule.rb
spec/services/automation_rules/action_service_spec.rb`
- `bundle exec rspec
spec/services/automation_rules/action_service_spec.rb`


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https://chatgpt.com/codex/tasks/task_e_6870c5f7b8b88326a9bd60b2ba710ccd

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/i18n/locale/en/automation.json

@@ -146,6 +146,7 @@
       "SEND_WEBHOOK_EVENT": "Send Webhook Event",
       "SEND_ATTACHMENT": "Send Attachment",
       "SEND_MESSAGE": "Send a Message",
+      "ADD_PRIVATE_NOTE": "Add a Private Note",
       "CHANGE_PRIORITY": "Change Priority",
       "ADD_SLA": "Add SLA",
       "OPEN_CONVERSATION": "Open conversation"

app/javascript/dashboard/routes/dashboard/settings/automation/constants.js

@@ -555,6 +555,11 @@ export const AUTOMATION_ACTION_TYPES = [
     label: 'SEND_MESSAGE',
     inputType: 'textarea',
   },
+  {
+    key: 'add_private_note',
+    label: 'ADD_PRIVATE_NOTE',
+    inputType: 'textarea',
+  },
   {
     key: 'change_priority',
     label: 'CHANGE_PRIORITY',