Remove UserId Obsolete Marker and Improve Certificate Mode Validation

- Removed the [Obsolete] marker from AlipayAuthenticationConstants.UserId while retaining explanatory comments.
- ClaimActions now directly maps “urn:alipay:user_id” to “user_id” and removes the obsolete mapping for Claims.UserId.
- Added non-null validation for PublicKey in certificate mode.

Author: AigioL

src/AspNet.Security.OAuth.Alipay/AlipayAuthenticationConstants.cs

@@ -36,7 +36,6 @@ public static class Claims
         /// The internal identifier for Alipay users will no longer be independently available going forward and will be replaced by OpenID.
         /// See https://opendocs.alipay.com/common/0ai736
         /// </summary>
-        [Obsolete("The internal identifier for Alipay users will no longer be independently available going forward and will be replaced by OpenID. See https://opendocs.alipay.com/common/0ai736")]
         public const string UserId = "urn:alipay:user_id";
     }
 }

src/AspNet.Security.OAuth.Alipay/AlipayAuthenticationOptions.cs

@@ -30,9 +30,9 @@ public AlipayAuthenticationOptions()
         ClaimActions.MapJsonKey(Claims.Nickname, "nick_name");
         ClaimActions.MapJsonKey(Claims.Province, "province");
         ClaimActions.MapJsonKey(Claims.OpenId, "open_id");
-#pragma warning disable CS0618
-        ClaimActions.MapJsonKey(Claims.UserId, "user_id");
-#pragma warning restore CS0618
+
+        // https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers/pull/1131#discussion_r2657531257
+        ClaimActions.MapJsonKey("urn:alipay:user_id", "user_id");
     }
 
     /// <summary>
@@ -79,6 +79,11 @@ public override void Validate()
             {
                 throw new ArgumentException($"The '{nameof(RootCertificateSnKeyId)}' option must be provided if the '{nameof(UseCertificateSignatures)}' option is set to true.", nameof(RootCertificateSnKeyId));
             }
+
+            if (PublicKey == null)
+            {
+                throw new ArgumentException($"The '{nameof(PublicKey)}' option must be provided if the '{nameof(UseCertificateSignatures)}' option is set to true.", nameof(PublicKey));
+            }
         }
     }
 }