Adds OPC UA SDK's CheckApplicationInstanceCertificate call to
OpcUaApplicationHost.StartAsync, removing the v1 friction of needing to
pre-create the PKI directory tree before booting.
- New OpcUaApplicationHostOptions.PkiStoreRoot (defaults to "pki")
- BuildConfigurationAsync now derives own/issuer/trusted/rejected from
PkiStoreRoot so the cert paths are configurable + consistent
- EnsureApplicationCertificateAsync runs before StandardServer.Start, and
fails fast with a clear message if the SDK can't produce a valid cert
- 2 new tests: fresh-tree creates a cert, second boot reuses it
Partial slice of follow-up F13. Endpoint-security, user-token validator,
and observability wiring still pending in the F13 follow-up. OpcUaServer
tests: 4 → 6.
