feat(dx): add distrobox container creation and app export integration test

[castrojo]

The distrobox integration test in tests/dx/features/dx_tools.feature only verifies
that the distrobox binary is present on PATH. The full integration - creating a
container, installing an app inside it, and exporting it to the host desktop - is
completely untested.

This is the core distrobox value proposition: mutable containers on an immutable
host. If the export mechanism breaks, users cannot use distrobox for GUI apps.

Proposed test scenarios:

1. Create a distrobox container: distrobox create --name test-box --image --yes
2. Install a CLI tool inside: distrobox enter test-box -- dnf install -y htop
3. Export to host: distrobox-export --bin /usr/bin/htop --export-path ~/.local/bin
4. Verify export: ls ~/.local/bin/htop exits 0

Blocker: The test requires a pre-cached container image to avoid Flathub/registry
timeouts in CI. Suggested image: registry.fedoraproject.org/fedora-toolbox:latest
This image should be pre-pulled into the VM or QEMU before tests run.

Tag proposed scenarios with @requires_cached_image until the caching infrastructure
is in place.

Related to the Flatpak caching effort - a similar pre-caching mechanism for OCI
container images would unblock this.

[castrojo]

Blocked prerequisite: the proposed distrobox integration needs a cached toolbox image and lab provisioning support. Testsuite can add the create/install/export scenarios after projectbluefin/lab provides a reliable image-cache path.