[snapshot] fix deprecated VS-connector subresources group

[kkozoriz]

Summary

VolumeSnapshot node subresources (manifests-download, manifests-with-data-restoration, manifests-and-children-refs-upload) are now served by storage-foundation, not state-snapshotter. d8 was still addressing the old group, which now works only as a server-side deprecated alias.

Problem

• The three subresources moved from state-snapshotter's subresources.snapshot.storage.k8s.io to storage-foundation's subresources.storage-foundation.deckhouse.io (version unchanged, still v1)
• The old group is kept only as a server-side alias (same handlers, byte-identical body, differs only in discovery name and a Warning: 299 header) — clients are not meant to rely on it
• d8's aggapi client was still hardcoded to the old group, with no fallback intended on either side

Fix

• VSConnectorGroup now points at subresources.storage-foundation.deckhouse.io; no dual-group or fallback logic added
• Split VolumeSnapshotVersion out of VSConnectorVersion: snapimport's CSI VolumeSnapshot RESTMapping call was borrowing the subresources group's version constant, which only worked because both happened to equal "v1"
• Updated the six expected paths and producer-reference comments in client_test.go, and the group name in the node.go comment

Before / After

Before: d8 snapshot download/upload/restore against a CSI VolumeSnapshot leaf addresses subresources.snapshot.storage.k8s.io.
After: the same commands address subresources.storage-foundation.deckhouse.io — same path shape, same body, only the group name changes.

Tests

• TestDownloadPath / TestSubresourcePath / TestUploadPath / TestAggregatedAPIContract — VS-leaf expectations updated to the new group
• TestConfig_VolumeSnapshotResource — new regression test pinning the literal CSI wire version, guards against reintroducing the VSConnectorVersion/VolumeSnapshotVersion mix-up

Notes

Manually verified against a live cluster: discovery serves the new group, and download/upload against a real CSI VolumeSnapshot leaf both succeed end-to-end.