refactor: use say_stream helper instead of client.chat_stream

[zimeg]

Summary

Replace manual client.chat_stream() calls with the say_stream context utility, which automatically injects channel, thread_ts, recipient_team_id, and recipient_user_id from the listener context.

This simplifies the message handler by removing the need to manually extract and pass these values from payload and context.

def message(
    logger: Logger,
    message: dict,
    say: Say,
    say_stream: SayStream,
    set_status: SetStatus,
):
    set_status(status="thinking...")
    streamer = say_stream(task_display_mode="plan")
    streamer.append(markdown_text="Hello!")
    streamer.stop()

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.