Handle MessageTooLarge error in router to prevent node crash

When a message exceeds gossipsub's 1 MiB limit, the resulting
RuntimeError("MessageTooLarge") was unhandled in _networking_publish,
causing the entire TaskGroup to fail and crash the node. Now catch
this error and log it, dropping the oversized message gracefully.

Fixes #1296

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

src/exo/routing/router.py

@@ -211,6 +211,14 @@ class Router:
                     pass
                 except AllQueuesFullError:
                     logger.warning(f"All peer queues full, dropping message on {topic}")
+                except RuntimeError as e:
+                    if "MessageTooLarge" in str(e):
+                        logger.error(
+                            f"Message too large for gossipsub on topic {topic} "
+                            f"({len(data)} bytes), dropping message"
+                        )
+                    else:
+                        raise
 
 
 def get_node_id_keypair(