A witness in a _test.go file fires on go test and go vet but NOT on go build
alone — and a build-only step is exactly how the R-88 Part 2 near-miss would have
shipped. Moved beside the type it pins, and added one for AgentVersionReporter.
No defect found: quiesceBackend and *Client both satisfy their interfaces today.
No version bump, no deploy — compile-time only.
