Fix structlog JSON serialization crash on non-serializable objects

Wrap the enc_hook in json_dumps() with a safe_default() fallback that
catches TypeError and falls back to str(). This prevents the logging
pipeline from crashing when log event dicts contain objects that
msgspec cannot serialize.

Closes: #62472
Closes: #62201

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

Author: Vamsi-klu

shared/logging/src/airflow_shared/logging/structlog.py

@@ -316,7 +316,17 @@ def json_dumps(msg, default):
                 "event": msg.pop("event"),
                 **msg,
             }
-            return msgspec.json.encode(msg, enc_hook=default)
+
+            def safe_default(obj):
+                """Fall back to str() if the default enc_hook fails or is None."""
+                if default is not None:
+                    try:
+                        return default(obj)
+                    except TypeError:
+                        pass
+                return str(obj)
+
+            return msgspec.json.encode(msg, enc_hook=safe_default)
 
         json = structlog.processors.JSONRenderer(serializer=json_dumps)