Drop KeyEncipherment from the self-signed cert's key-usage extension — it
is semantically wrong for ECDSA (RSA key-transport only); DigitalSignature
alone is correct for TLS 1.3 / ECDHE server certs. CertificateRequest is
unchanged (not IDisposable in .NET 10). Test now also asserts MachineName,
127.0.0.1 and IPv6 loopback are present in the SAN extension.