Forward SELinux relabel flags for non-bind mounts

[Sanjays2402]

Closes #1516

Contributor Checklist:

Please make sure to read development guidelines in CONTRIBUTING.md.

Compose spec for the short volume syntax the relabel flag comes from:
https://github.com/compose-spec/compose-spec/blob/main/spec.md#short-syntax-5

For any user-visible change please add a release note to newsfragments directory.

I could not add the newsfragments/*.bugfix file from my side; please tell me if you want me to
push one, or feel free to add it with this text: "The SELinux relabel flags z and Z given in
the short volume syntax are no longer dropped for named volumes when the mount is passed to podman
as --mount."

All changes require additional unit tests.

Added test_selinux_named_volume_short_syntax in tests/unit/test_container_to_args.py, next to
the existing test_selinux_volume. It fails on main (the ,Z is missing from the generated
--mount argument) and passes with the fix; python -m unittest discover tests/unit is green
(480 tests), and ruff format --check / ruff check are clean on both changed files.

---

parse_short_mount stores the z/Z of db:/var/lib/mysql:Z as a bind propagation option. Since
--mount became the default for named volumes, mount_desc_to_mount_args() only read the selinux
option for bind mounts, so the relabel flag was silently dropped — which is the 1.5.0 → 1.6.0
regression reported in #1516. This forwards z/Z for the other mount types too, mirroring what
mount_desc_to_volume_args() already does for the -v syntax.

This change was prepared with AI assistance; the regression test was run locally and fails without the fix.