Add grpcserver test coverage

- Add comprehensive test file for internal/grpcserver package
- Cover interceptors, system, engine, policy, and auth handlers
- Cover pbToRule/ruleToPB conversion helpers
- 37 tests total; CA/PKI/ACME and Login/Logout skipped (require live deps)

Co-authored-by: Junie <junie@jetbrains.com>

internal/config/config.go

@@ -21,7 +21,7 @@ type Config struct {
 // ServerConfig holds HTTP/gRPC server settings.
 type ServerConfig struct {
 	ListenAddr  string `toml:"listen_addr"`
-	GRPCAddr    string `toml:"grpc_addr"`
+	GRPCAddr string `toml:"grpc_addr"`
 	TLSCert     string `toml:"tls_cert"`
 	TLSKey      string `toml:"tls_key"`
 	ExternalURL string `toml:"external_url"` // public base URL for ACME directory, e.g. "https://metacrypt.example.com"