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Issue body

Message id is useful to correlate ModelClientStreamingChunkEvent with the eventual completed message being produced.

This should be added to BaseChatMessage and BaseAgentEvent as an id: str field. For ModelClientStreamingChunkEvent, it should have a separate optional field full_message_id: str | None = None to reference the complete message that may come after the chunks. So the consumer of the stream don't need to double-print it.