What this PR does / why we need it:

The memory driver parsed storage keys with strings.Split(key, ".v") and required exactly two parts. Release names may contain dots, so a valid name such as my.vault or app.v2 made Get and Delete return release: invalid key, while Create and Query worked.

This moves the key parsing into a small helper that splits on the last .v and still validates that the suffix is a number. The same change is applied to the copy in internal/storage/driver.

Repro before the change:

HELM_DRIVER=memory HELM_MEMORY_DRIVER_DATA=rels.yaml helm get values my.vault --revision 1
Error: release: invalid key

Special notes for your reviewer:

Keys without a numeric version suffix are still rejected with ErrInvalidKey.

If applicable:

• this PR contains user facing changes (the docs needed label should be applied if so)
• this PR contains unit tests
• this PR has been tested for backwards compatibility

Fixes #32781