posix: Using bp::std_out in conjunction with bp::limit_handles

Boost version: 1.72.0
Platform: Linux

I use `bp::limit_handles` in order to avoid leaking file descriptors to child processes. This works fine, but I was unsuccessful combining this with redirection of `std_out` to a `string` `future`.
```
/* works fine */
std::vector stdOut(4096);
auto child = bp::system("/bin/echo", "1",
bp::std_out > boost::asio::buffer(stdOut),
bp::limit_handles);

/* string is empty */
std::future stdOut;
auto child = bp::system("/bin/echo", "1",
bp::std_out >stdOut,
bp::limit_handles);

```
In the second example, the `future` is valid, but the `string` is empty. When I remove `bp::limit_handles`, `stdOut` contains `"1\n"`.

I recognized that there is a slight difference between `async_out_buffer` ...
```
template
struct async_out_buffer : ::boost::process::detail::posix::handler_base_ext,
::boost::process::detail::posix::require_io_context,
::boost::process::detail::uses_handles
{
...
```
... and `async_out_future`:
```
template
struct async_out_future : ::boost::process::detail::posix::handler_base_ext,
::boost::process::detail::posix::require_io_context
{
...
```

While the former derives from `bp::detail::use_handles` (which seems to avoid closing this file descriptor), the latter doesn't do this.

Is this a by intention or a bug? Is there a way redirecting `std_out` to a `string` together with closing all other file handles?

**Side question:** How can I determine the number of output bytes for the first example (with the vector)?

regards
Christian