Migrate from github.com/golang/mock to go.uber.org/mock

Migrate off github.com/golang/mock, which Google archived in June 2023, to the maintained fork go.uber.org/mock. The fork's gomock API is a superset of the original's, so this is a path swap: imports, //go:generate mockgen directives and the go.mod require all move, and call sites are untouched. Run RemoveRedundantGomockFinish afterwards to drop the defer ctrl.Finish() calls the fork makes unnecessary, and go mod tidy to sync go.sum.