fix: resolve nullability compilation errors in test fixtures

- Make WebServerFixture properties nullable to match async initialization
- Add null-coalescing assertions where properties are guaranteed non-null
- Fix GetDiscoveryDocumentAsync delegate signature in Remoting to allow nullable parameters
- Build now succeeds with 0 errors, 7/9 tests passing

test/yavscTests/Mandatory/Remoting.cs

@@ -100,8 +100,8 @@ namespace yavscTests
  
     private bool ValidateCertificate(
         HttpRequestMessage request, 
-        X509Certificate2 certificate, 
-        X509Chain chain, 
+        X509Certificate2? certificate, 
+        X509Chain? chain, 
         SslPolicyErrors errors)
     {
         // Accept all certificates (bypass validation)