Make OpenSamlAssertingPartyDetails Serializable

Closes gh-17622

config/src/test/java/org/springframework/security/SerializationSamples.java

@@ -171,12 +171,14 @@ import org.springframework.security.saml2.provider.service.authentication.Saml2A
 import org.springframework.security.saml2.provider.service.authentication.Saml2AuthenticationToken;
 import org.springframework.security.saml2.provider.service.authentication.Saml2PostAuthenticationRequest;
 import org.springframework.security.saml2.provider.service.authentication.Saml2RedirectAuthenticationRequest;
+import org.springframework.security.saml2.provider.service.authentication.TestOpenSamlObjects;
 import org.springframework.security.saml2.provider.service.authentication.TestSaml2AuthenticationTokens;
 import org.springframework.security.saml2.provider.service.authentication.TestSaml2Authentications;
 import org.springframework.security.saml2.provider.service.authentication.TestSaml2LogoutRequests;
 import org.springframework.security.saml2.provider.service.authentication.TestSaml2PostAuthenticationRequests;
 import org.springframework.security.saml2.provider.service.authentication.TestSaml2RedirectAuthenticationRequests;
 import org.springframework.security.saml2.provider.service.authentication.logout.Saml2LogoutRequest;
+import org.springframework.security.saml2.provider.service.registration.OpenSamlAssertingPartyDetails;
 import org.springframework.security.saml2.provider.service.registration.RelyingPartyRegistration;
 import org.springframework.security.saml2.provider.service.registration.TestRelyingPartyRegistrations;
 import org.springframework.security.web.PortResolverImpl;
@@ -522,6 +524,11 @@ final class SerializationSamples {
 			return token;
 		});
 		generatorByClassName.put(Saml2LogoutRequest.class, (r) -> TestSaml2LogoutRequests.create());
+		generatorByClassName.put(OpenSamlAssertingPartyDetails.class,
+				(r) -> OpenSamlAssertingPartyDetails
+					.withEntityDescriptor(
+							TestOpenSamlObjects.entityDescriptor(TestRelyingPartyRegistrations.full().build()))
+					.build());
 
 		// web
 		generatorByClassName.put(AnonymousAuthenticationToken.class, (r) -> {

saml2/saml2-service-provider/src/main/java/org/springframework/security/saml2/provider/service/registration/OpenSamlAssertingPartyDetails.java

@@ -16,6 +16,7 @@
 
 package org.springframework.security.saml2.provider.service.registration;
 
+import java.io.Serial;
 import java.security.cert.CertificateException;
 import java.security.cert.X509Certificate;
 import java.util.ArrayList;
@@ -46,7 +47,10 @@ import org.springframework.security.saml2.core.Saml2X509Credential;
  */
 public final class OpenSamlAssertingPartyDetails extends RelyingPartyRegistration.AssertingPartyDetails {
 
-	private final EntityDescriptor descriptor;
+	@Serial
+	private static final long serialVersionUID = -2412785556799182734L;
+
+	private final transient EntityDescriptor descriptor;
 
 	OpenSamlAssertingPartyDetails(RelyingPartyRegistration.AssertingPartyDetails details, EntityDescriptor descriptor) {
 		super(details.getEntityId(), details.getWantAuthnRequestsSigned(), details.getSigningAlgorithms(),