A container for a streamed message delta and its termination signal.

During streaming, delta carries the incremental payload; once a terminal condition is reached, finish_reason may be populated to explain why.

Examples

let mut out = MessageOutput::new();
out.delta = MessageDelta::new().with_role(Role::Assistant).with_contents([PartDelta::Text { text: "Hi".into() }]);
assert!(out.finish_reason.is_none());
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Lifecycle

• While streaming: finish_reason is typically None.
• On completion: finish_reason is set; callers can then finish() the delta to obtain a concrete [Message].