fix(cli): add missing os + path/filepath imports to client_test.go

Follow-up to 0fb7d46. TestClient_ImportCertificates_SixFieldPayload
uses filepath.Join(t.TempDir(), ...) and os.WriteFile to stage a
test PEM, but the import block only listed encoding/json,
encoding/pem, net/http, etc. — neither os nor path/filepath was
imported. go vet rejected the package with 'undefined: filepath'
(and would have caught 'undefined: os' next).

Add both imports. No behavioral change — the referenced symbols
are the standard library's usual names for their respective
packages, so the test compiles and runs exactly as intended.
CI should now pass go build + go vet on the cli package.

internal/cli/client_test.go

@@ -10,6 +10,8 @@ import (
 	"math/big"
 	"net/http"
 	"net/http/httptest"
+	"os"
+	"path/filepath"
 	"testing"
 	"time"
 )