Function spawn_runtime_command 

pub fn spawn_runtime_command(
    command: Command,
    runtime_name: &str,
) -> Result<(Child, ChildStdin, ChildStdout), AppServerError>

Spawns one app-server child process with piped stdin/stdout and hidden stderr, returning the child plus owned stdio handles.

Runtime bootstraps require line-delimited JSON-RPC over stdin/stdout, no interactive stderr stream, and kill_on_drop(true) so abandoned runtimes do not leak.

Errors

Returns a provider error when the command cannot be spawned or either required stdio pipe is unavailable.