Claude Messages SSE event data; transport framing is deliberately separate.
Sources: Streaming Messages.md; Beta types.md RawMessageStreamEvent and deltas.
Optional/nullability omitted by the rendered Beta table is corroborated by
anthropic-sdk-python types/beta/{beta_message,beta_raw_message_delta_event,
beta_message_delta_usage,beta_compaction_content_block_delta}.py.