Function _stream_response

Stream response tokens to stdout in real-time. Uses StreamState to track cross-event context (plan progress, output_conclusion accumulation, tool_call_response lifecycle). Returns (got_content: bool, need_user_confirm: bool, output_text: str).

(
    message_handler: MessageHandler,
    session,
    query: str,
    data_source: Optional[DataSource] = None,
    output_mode: str = "summary",
    output_dir: Optional[Path] = None,
    process_log_handler: Optional[StructuredLogHandler] = None,
    is_attach: bool = False,
)