use metadata from SessionMessage to propagate related_request_id (#591)

tests/client/test_logging_callback.py

@@ -9,7 +9,6 @@ from mcp.shared.memory import (
 from mcp.shared.session import RequestResponder
 from mcp.types import (
     LoggingMessageNotificationParams,
-    NotificationParams,
     TextContent,
 )
 
@@ -80,10 +79,7 @@ async def test_logging_callback():
         assert log_result.isError is False
         assert len(logging_collector.log_messages) == 1
         # Create meta object with related_request_id added dynamically
-        meta = NotificationParams.Meta()
-        setattr(meta, "related_request_id", "2")
         log = logging_collector.log_messages[0]
         assert log.level == "info"
         assert log.logger == "test_logger"
         assert log.data == "Test log message"
-        assert log.meta == meta