rpc: answer GetWorkload for a missing workload with codes.NotFound

[CMGS]

Follow-up to #756. This lets clients recognise a missing workload from the gRPC status instead of from the error text.

GetWorkload returns codes.NotFound

• When the error chain contains types.ErrWorkloadNotExists, GetWorkload now answers codes.NotFound. Every other failure keeps the per-RPC code GetWorkload (1051).
• Before this change, agent's WorkloadExists had to string-match bad \Count` valueorworkload not existsin the status message. The companion agent PR switches it tostatus.Code(err) == codes.NotFound`.

The ErrInvaildCount mark from #756 is removed

• An etcd read that matches zero keys now wraps only ErrKeyNotFound.
• The store: report a missing etcd key as key not found #756 description said resource-extend's nodestore.CheckAbsent needed the mark. That was wrong. nodestore reads through the raw etcd client and raises its own ErrNodeNotExists on zero matches, so it never sees core's GetOne errors.
• A search of the projecteru2 org for ErrInvaildCount finds no other consumer of core's not-found error. The companion resource-extend PR also cleans up its side.

Deploy order

• Roll out core before the new agent.
• An old agent against a new core keeps working, because the message still contains workload not exists.
• A new agent against an old core would see the custom code and report the workload as existing.

Tests

• TestGetWorkloadReportsAMissingWorkloadAsNotFound checks that a missing workload maps to codes.NotFound and that any other error keeps code 1051.
• The etcd missing-key test now asserts the error is not ErrInvaildCount.

Gates (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.