Function run_agent_turn_joining 

pub async fn run_agent_turn_joining(
    deps: &AgentTurnDeps,
    block_id: String,
    message: String,
    message_id: Option<String>,
    registration: TurnRegistration,
    origin: TurnOrigin,
    attachments: Vec<AttachmentRef>,
    joins_turn: Option<u64>,
) -> Result<(), String>

run_agent_turn for a message the pane held while turn joins_turn ran: if that turn is still settling (or only just ended), the message joins it rather than starting a new one (turn-model spec §4.3, J3).