nodestore: an absent node is ErrNodeNotExists and nothing else

[CMGS]

This is the resource-extend side of the not-found cleanup started in projecteru2/core#756 and projecteru2/core#757.

Two problems fixed

• Store.Get reads one exact etcd key without WithPrefix. etcd answers that read with at most one match, so default: ErrInvaildCount was unreachable. Get now has two outcomes: zero matches returns ErrNodeNotExists, and one match returns the decoded value.
• CheckAbsent treated ErrInvaildCount as "absent". If that branch had ever fired, it would have hidden an inconsistency behind a green node add. CheckAbsent now accepts only ErrNodeNotExists.

Dependencies

• nodestore never depended on core's GetOne errors, because it uses the raw etcd client. No core bump is needed.
• go.etcd.io/etcd/api/v3 moves from indirect to direct at the same version, because the new test builds mvccpb.KeyValue.

Tests
This adds the package's first tests. TestGetAndCheckAbsent uses a fake KV and covers three cases for both Get and CheckAbsent: node absent, node present, and a store error that propagates.

Gates, all run with GOWORK=off:

• make fmt-check passes.
• make lint reports 0 issues on linux and darwin.
• asl is clean on linux and darwin.
• make test and make build pass.