fix: OrderedDict has no .add(), use key assignment for delivery dedup

Commit 75a6d36 changed processed_deliveries from set to OrderedDict
for deterministic eviction, but mark_delivery_processed() still used
.add() which is a set-only method. Changed to key assignment:
processed_deliveries[delivery_id] = None

Fixes 6 failing tests in CI (AttributeError on main).

Author: manascb1344

runner/core/validation.py

@@ -61,7 +61,7 @@ def is_duplicate_delivery(delivery_id: str) -> bool:
 def mark_delivery_processed(delivery_id: str):
     """Mark a delivery ID as processed for replay protection."""
     cleanup_delivery_cache()
-    processed_deliveries.add(delivery_id)
+    processed_deliveries[delivery_id] = None
 
 
 def reset_caches():