gRPC AuthContext: SAN dNSName extraction relies on parsing X509Extension.Format() text instead of typed API

[`X509CertificateHelpers.GetDnsFromExtensions`](https://github.com/dotnet/aspnetcore/blob/67a02a66a21ac4f2e8ab19201b59cf45e97c8c98/src/Grpc/JsonTranscoding/src/Shared/X509CertificateHelpers.cs#L38) extracts SAN `dNSName` values by calling `X509Extension.Format(false)` and string-splitting the result, instead of parsing the extension's structured ASN.1 content. `Format()` returns a human-readable, OS/culture-dependent string, and a `dNSName` value could itself contain characters like `,` or `=` that collide with the delimiters this code assumes (e.g. `", "`, `"DNS Name="`/`"DNS:"`). Since .NET 9, `X509SubjectAlternativeNameExtension.EnumerateDnsNames()` parses the extension's DER content directly and should be used instead.

Start in src/Grpc/JsonTranscoding/src/Shared/X509CertificateHelpers.cs at GetDnsFromExtensions, then review X509SubjectAlternativeNameExtension. Replace the text-based SAN extraction with the typed EnumerateDnsNames() API described in the issue. Done means DNS names are obtained from the extension's structured content without relying on Format() or delimiter parsing.

Written by the indexing model from the issue text.