feat: Add a priority + created at sort for conversations (#13658)

- Add a new conversation sort option "Priority: Highest first, Created:
Oldest first" that sorts by priority descending (urgent > high > medium
> low > none) with created_at ascending as the tiebreaker

app/finders/conversation_finder.rb

@@ -11,6 +11,7 @@ class ConversationFinder
     'priority_desc' => %w[sort_on_priority desc],
     'waiting_since_asc' => %w[sort_on_waiting_since asc],
     'waiting_since_desc' => %w[sort_on_waiting_since desc],
+    'priority_desc_created_at_asc' => %w[sort_on_priority_created_at desc],
 
     # To be removed in v3.5.0
     'latest' => %w[sort_on_last_activity_at desc],