Represents the message_delta SSE event from the Anthropic streaming API.
Emitted after all content blocks have closed. Carries the stop_reason
(:end_turn or :tool_use) and final usage statistics (output tokens).
Summary
Types
@type stop_reason() :: :end_turn | :tool_use | String.t() | nil
@type t() :: %Anthropic.Event.MessageDelta{ stop_reason: stop_reason(), usage: map() | nil }