Remove unnecessary Vec allocation in debug log

HashMap::keys() already implements Debug, no need to collect into a Vec.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-workflows/src/handler/manager_loop.rs

@@ -189,7 +189,7 @@ impl Handler for SubWorkflowHandler {
 
                     tracing::debug!(
                         node = %node.id,
-                        propagated_keys = ?diff.keys().collect::<Vec<_>>(),
+                        propagated_keys = ?diff.keys(),
                         "Sub-workflow context diff filtered"
                     );