S3 upload metadata regressed: `platform_variant` dropped from `meta/singles/*` after refactor

[nkraetzschmar]

Summary

During the refactor that switched S3 upload to load metadata via CName.load_from_release_file(), the platform_variant field stopped being written into the YAML metadata uploaded to meta/singles/{cname}.

This field was introduced in ce32bb0 and should not have been dropped.

Why this matters

platform_variant is required metadata to distinguish platform sub-variants (e.g. for openstack/openstack-metal splits). Without it, we cannot reliably interpret uploaded artifacts based on the metadata alone.

Regression details

Before the refactor:

• upload_from_directory() read GARDENLINUX_PLATFORM_VARIANT from the release file and wrote metadata["platform_variant"] = ... when present.

After the refactor:

• upload_from_directory() builds metadata from cname_object fields but never re-adds platform_variant (even though CName.load_from_release_file() can read/cache it).

Result: meta/singles/* no longer contains platform_variant.

Expected behavior

If GARDENLINUX_PLATFORM_VARIANT is present in the release file, the uploaded YAML metadata must include:

• platform_variant: <value>

This should be restored in the current S3 upload flow.