Decoder for Anthropic Messages API SSE streams (stream: true).

stream_event/1 converts a decoded Anthropic event json map into a list of normalized stream events:

• {:text, binary} — content_block_delta / text_delta
• {:thinking, binary} — content_block_delta / thinking_delta
• {:tool_call, map} — content_block_start tool_use (id/name) and content_block_delta / input_json_delta (partial json), tagged with the block index
• {:usage, map} — normalized usage; message_start carries input tokens, message_delta the final output tokens
• {:finish, atom} — normalized stop reason from message_delta

ping and unknown event types decode to [].