feat: add automation condition to filter private notes (#12102)

## Summary

Adds a new automation condition to filter private notes.

This allows automation rules to explicitly include or exclude private
notes instead of relying on implicit behavior.

Fixes: #11208 

## Preview



https://github.com/user-attachments/assets/c40f6910-7bbf-4e59-aae5-ad408602927a

spec/models/automation_rule_spec.rb

@@ -86,6 +86,19 @@ RSpec.describe AutomationRule do
       rule = FactoryBot.build(:automation_rule, params)
       expect(rule.valid?).to be true
     end
+
+    it 'allows private_note as a valid condition attribute' do
+      params[:conditions] = [
+        {
+          attribute_key: 'private_note',
+          filter_operator: 'equal_to',
+          values: [true],
+          query_operator: nil
+        }
+      ]
+      rule = FactoryBot.build(:automation_rule, params)
+      expect(rule.valid?).to be true
+    end
   end
 
   describe 'reauthorizable' do