Represents the content_block_start SSE event from the Anthropic streaming API.
Emitted when a new content block opens (text or tool-use). The index
identifies which block in the message content array is starting, and
content_block describes its type and any initial metadata.
Summary
Types
@type t() :: %Anthropic.Event.ContentBlockStart{ content_block: content_block(), index: non_neg_integer() }