userns_mode option is not honoured

Hi,

It seems like userns mode is not honoured by either podman-py or the podman API.

Running podman 4.9.3 on an up-to-date Ubuntu 24.04.1 LTS virtual machine.

```
$ cat /etc/sub{uid,gid}
[some random username]:100000:65536
containers:2147483647:2147483648
[some random username]:100000:65536
containers:2147483647:2147483648
```

When creating and running the same exact container with `podman-py` and with the Podman CLI, it is apparent that in the former case the `userns_mode='auto'` option is not honoured.

I attach this test case which shows the issue clearly: https://gist.github.com/vmsh0/f32b137b6d4bf1d3dbf5b6745e92911c

One unit test creates a container using `podman-py`: the test fails as after inspecting the container you can see that `UsernsMode` is empty. The other unit test creates the same container by calling the podman CLI: in this case, the test succeeds and you can see that `UsernsMode` is set to `'private'` as expected.

As far as I can see, this fails silently. I noticed the issue by pure chance while investigating a different problem. This might potentially have security implications, as users might be expecting their containers to run in a separate user namespace while they are really running in the root namespace.

Start with the attached gist and its two unit tests, comparing the podman-py container-creation path with the Podman CLI invocation. Trace how userns_mode='auto' is sent through the API and inspect the resulting UsernsMode value. Done means the podman-py case honors the option like the CLI case and the regression tests pass.

Written by the indexing model from the issue text.