Function handle_exec_command

crates/openshell-tui/src/lib.rs:984–1128

Suspend the TUI, execute a command on a sandbox via SSH, then resume. Mirrors `handle_shell_connect` but passes the user's command to SSH instead of opening an interactive shell. The TUI is suspended while the command runs; press Ctrl-C to stop and return to the TUI.

(
    app: &mut App,
    terminal: &mut Terminal<CrosstermBackend<io::Stdout>>,
    events: &EventHandler,
    sandbox_name: &str,
    command: &str,
)

/// instead of opening an interactive shell.  The TUI is suspended while
/// the command runs; press Ctrl-C to stop and return to the TUI.
async fn handle_exec_command(
    app: &mut App,
    terminal: &mut Terminal<CrosstermBackend<io::Stdout>>,
    events: &EventHandler,
    sandbox_name: &str,
    command: &str,
) {
    // Step 1: Resolve sandbox → SSH session (same as handle_shell_connect).
    let sandbox_id = {
        let req = openshell_core::proto::GetSandboxRequest {
            name: sandbox_name.to_string(),
        };
        match tokio::time::timeout(Duration::from_secs(5), app.client.get_sandbox(req)).await {
            Ok(Ok(resp)) => {
                if let Some(s) = resp.into_inner().sandbox {
                    s.object_id().to_string()
                } else {
                    app.status_text = format!("exec: sandbox {sandbox_name} not found");
                    return;
                }
            }
            Ok(Err(e)) => {
                app.status_text = format!("exec: failed to get sandbox: {}", e.message());
                return;
            }
            Err(_) => {
                app.status_text = "exec: get sandbox timed out".to_string();
                return;
            }
        }
    };

    let session = {
        let req = openshell_core::proto::CreateSshSessionRequest {
            sandbox_id: sandbox_id.clone(),
        };
        match tokio::time::timeout(Duration::from_secs(5), app.client.create_ssh_session(req)).await
        {
            Ok(Ok(resp)) => resp.into_inner(),
            Ok(Err(e)) => {
                app.status_text = format!("exec: SSH session failed: {}", e.message());
                return;
            }
            Err(_) => {
                app.status_text = "exec: SSH session timed out".to_string();
                return;
            }
        }
    };
    if let Err(err) = validate_ssh_session_response(&session) {
        app.status_text = format!("exec: gateway returned invalid SSH session response: {err}");
        return;
    }

    // Step 2: Resolve gateway and build ProxyCommand (same as handle_shell_connect).
    #[allow(clippy::cast_possible_truncation)]
    let gateway_port_u16 = session.gateway_port as u16;
    let (gateway_host, gateway_port) =