feat(ocpp): auto-derive signing method from EC public key curve

feat(ocpp): auto-derive signing method from EC public key curve

- Add deriveSigningMethodFromPublicKeyHex that extracts EC curve OID
  from ASN.1 DER public key and maps to SigningMethodEnumType
- Add validateSigningPrerequisites that validates public key presence,
  curve detection, and config consistency — returns enabled/disabled
  with reason
- Integrate validation in OCPP 2.0 (OCPPServiceUtils.buildMeterValue)
  and OCPP 1.6 (readSigningConfigForConnector) signing paths
- On validation failure: log error and gracefully fallback to unsigned
  meter values instead of producing invalid signed data
- Move TEST_PUBLIC_KEY_HEX to ChargingStationTestConstants for reuse
- Update test fixtures to use valid secp256k1 ASN.1 EC public key