fix(ocpp20): implement proper M04.FR.06 guard via isChargingStationCertificateHash

fix(ocpp20): implement proper M04.FR.06 guard via isChargingStationCertificateHash

The previous M04.FR.06 guard was ineffective: it called
getInstalledCertificates() which scans root cert directories and
maps types via mapInstallTypeToGetType(), which has no case for
ChargingStationCertificate (stored via CertificateSigned, not
InstallCertificate). The V2GCertificateChain filter never matched.

Add isChargingStationCertificateHash() to OCPP20CertificateManager
that directly scans the ChargingStationCertificate directory and
compares certificate hashes. Use it in handleRequestDeleteCertificate
for a reliable M04.FR.06 guard.