feat: move MockLogger into a logtest sub-package

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>

log_test.go

@@ -23,11 +23,3 @@ func TestContextLogger(t *testing.T) {
 	ctx := ContextWithLogger(context.Background(), custom)
 	assert.Same(t, custom, LoggerFromContext(ctx))
 }
-
-func TestMockLogger(t *testing.T) {
-	m := NewMockLogger()
-	m.Logger().Info("hello", "k", "v")
-	m.Check(t, []string{`level=INFO msg=hello k=v`})
-	empty := NewMockLogger()
-	empty.Check(t, nil)
-}