docs: fix stale API references in relay.md

[PHILLIPS71]

What's wrong

IIdSerializer doesn't exist anymore. It was removed in df07a9169 (#7026, "Added new node id serializer.", April 2024) along with IdSerializer, IdMiddleware, and IdValue, and replaced with INodeIdSerializer.

INodeIdSerializer also has a different shape than the old interface. It's Format(string typeName, object internalId), not Serialize(schemaName, typeName, id):

src/HotChocolate/Core/src/Types/Types/Relay/Serialization/INodeIdSerializer.cs:6-20

public interface INodeIdSerializer
{
    string Format(string typeName, object internalId);
    ...
}

The old three-argument Serialize(null, "Product", productId) call has no direct equivalent, Format drops the schema name entirely.

While I was in there I checked the rest of the page against source and found three more things that had drifted:

1. AddQueryFieldToMutationPayloads generates query: Query!, not query: Query. The field is non-null:

   src/HotChocolate/Core/src/Types/Types/Relay/QueryFieldTypeInterceptor.cs:44-49

   var queryFieldType = TypeReference.Parse($"{_queryType.Name}!");

2. The predicate example called .Value on a plain string. MutationPayloadPredicate is Func<ITypeDefinition, bool>, and Name on ITypeDefinition is just a string. The framework's own default predicate shows the right usage:

   src/HotChocolate/Core/src/Types/Types/Relay/MutationPayloadOptions.cs:17-18

   public Func<ITypeDefinition, bool> MutationPayloadPredicate { get; set; } =
       type => type.Name.EndsWith("Payload", StringComparison.Ordinal);

3. The NodeIdValueSerializer source generator doesn't fire automatically. The doc implied it kicks in whenever you use [ID] on a custom type. It actually only triggers on an explicit call to AddNodeIdValueSerializerFrom<T>():

   src/HotChocolate/Core/src/Types.Analyzers/Inspectors/NodeIdValueSerializerInspector.cs:19-29