NewMockLogger/MockLogger move from the main `logging` package to a new
`logging/logtest` sub-package, so the production `logging` package's import
graph no longer pulls in testify — only consumers that import logtest do.
testify stays in the module's go.mod (used by logtest and the library's own
tests).
Breaking import change for consumers: `logging.NewMockLogger` becomes
`logtest.NewMockLogger`. The backend services that reference it are updated to
the logtest import alongside their go.mod bump to this release.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
