Fix: Use default_factory for metadata in Message

[aleLoPrete]

• Bug: Message dataclass was using a mutable default value for the metadata field, preventing the spray-and-wait router from adding messages correctly:

  • ValueError: mutable default <class 'dict'> for field metadata is not allowed: use default_factory

• Fix: The metadata field now uses default_factory=dict to ensure each Message instance receives its own independent dictionary.