SIENTIAPDE-1973: Implement per-connector OPC client certificates and remove legacy read engine.
This update overhauls certificate management, transitioning from a single platform certificate to individual client certificates defined in the slot configuration. The CertProvider now downloads certificates from MinIO based on certificate_id, applying dynamic security policies (mode, policy) from the slot. The legacy READ_ENGINE=legacy option and OpcManager have been entirely removed, standardizing on the ServerReader with strict slot v2 enforcement. Write command resolution (ServerConfigResolver) now consistently uses connectors joined with opc_certificates. The CERT_BUCKET environment variable is no longer used.
