* test: pin that an unusable version id is refused, never resolved
A version id containing a path separator, or "." / "..", can never name a
stored version. Resolving one to the null or latest version instead would let
a caller destroy a live version by asking for one that does not exist, on a
bucket configured for immutability.
The guard exists today and holds on every verb; it had no test. Pins two
properties: such a request is refused with a client error rather than a 5xx
(a 5xx invites endless retries of something that can never succeed), and the
version that does exist survives every refused request.
* test: require exactly 400 for an unusable version id
