fix(drain): msg.metadata is a property, not a method

nats-py's Msg.metadata is a property returning a Metadata object
directly — not an async callable. Removes the erroneous () call.

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

meshai/central/consumer.py

@@ -687,7 +687,7 @@ class CentralConsumer:
         # while we still hold the message (prevents interleaving).
         if self._draining:
             try:
-                meta = await msg.metadata()
+                meta = msg.metadata
                 if meta is not None and getattr(meta, "num_pending", None) == 0:
                     self._drain_complete()
             except Exception: