Analyzer: warn when SslCertificateTrust is set without a RemoteCertificateValidationCallback

On the raw callback path (`TlsHandshakeCallbackOptions.OnConnection` returning a `SslServerAuthenticationOptions`), setting `ServerCertificateContext` with an `SslCertificateTrust` looks like it restricts which client certs are accepted. It doesn't. `SslCertificateTrust` only fills the `certificate_authorities` list in the TLS `CertificateRequest` — it's a hint to the client, not a validation gate.

Without a `RemoteCertificateValidationCallback`, a client can ignore the advertised CAs and present any cert chaining to a system-trusted root. This bites hardest on HTTP/3, where the enforcement behavior of MsQuic differs from SslStream, so an app that "worked" on H1/H2 can silently accept unintended client certs over H3.

related https://github.com/dotnet/runtime/issues/73053

## Proposal

Add a Roslyn analyzer that flags the misconfiguration at build time.

Rule: if `SslCertificateTrust` is passed to `SslStreamCertificateContext.Create(...)` (or `ServerCertificateContext.Trust` is otherwise set) **and** `ClientCertificateRequired` is true, **and** no `RemoteCertificateValidationCallback` is assigned on the same `SslServerAuthenticationOptions`, emit a warning.

- Category: Security
- Severity: Warning
- Message: "A client-certificate trust list is set but no RemoteCertificateValidationCallback is configured. Add a validation callback that checks the client chain against your roots."
- Code fix: insert a `RemoteCertificateValidationCallback` stub that builds an `X509Chain` with `X509ChainTrustMode.CustomRootTrust`.

Start by locating the analyzer entry point for SslStreamCertificateContext.Create and SslServerAuthenticationOptions. Trace how ServerCertificateContext.Trust, ClientCertificateRequired, and RemoteCertificateValidationCallback are represented, then add the warning and code fix described; done means the specified misconfiguration is diagnosed with the stated message and a callback stub is offered.

Written by the indexing model from the issue text.