`DataContractSerializer`: Serialize child class as base class

### Description

I have following on my server:

```C#
[DataContract]
public class Base { }

[OperationContract]
void DoSth(Base base) { }
```

and on my client:

```C#
class Child : Base { }
```

When calling the server with `DoSth(child)` I get a serialization exception, because `Child` is not known to the WCF DataContractSerializer:

```
System.ServiceModel.CommunicationException: There was an error while trying to serialize parameter http://tempuri.org/:base. The InnerException message was 'Type 'Child' with data contract name 'Child:http://schemas.datacontract.org/2004/07/Client' is not expected. Add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.'. Please see InnerException for more details.
---> System.Runtime.Serialization.SerializationException: Type 'Child' with data contract name 'Child:http://schemas.datacontract.org/2004/07/Client' is not expected. Add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
```
And I don't want to make it known, because the server uses only `Base`'s properties. How can I send my data as `Base object`?

I know, I could create a new `Base` object and copy all properties, but this is not really elegant.

see also https://stackoverflow.com/questions/78061526/datacontractserializer-deserialize-child-class-into-base-class

### Reproduction Steps

* Create WCF server and client.
* Add `Base` and `Child` classes as described above.
* Send instance of `Child` to server.

### Expected behavior

`Child` should be serialized as `Base`, ignoring all additional properties.

### Actual behavior

```
System.ServiceModel.CommunicationException: There was an error while trying to serialize parameter http://tempuri.org/:base. The InnerException message was 'Type 'Child' with data contract name 'Child:http://schemas.datacontract.org/2004/07/Client' is not expected. Add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.'. Please see InnerException for more details.
---> System.Runtime.Serialization.SerializationException: Type 'Child' with data contract name 'Child:http://schemas.datacontract.org/2004/07/Client' is not expected. Add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
```

### Regression?

_No response_

### Known Workarounds

_No response_

### Configuration

.NET 8 on Windows 11, but should not be specific to that environment.

### Other information

_No response_