[Bug]: DeSerializeRequest throws NotSupportedException for an unrecognised @type #400

What type of issue is this?

• Bug report
• Feature request

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the SysML2.NET
• I have searched open and closed issues to ensure it has not already been reported

Description

When a request body carries an @type that the request family does not define (e.g. "BranchRequest" instead of "Branch"), DeSerializer.DeSerializeRequest<T> / DeSerializeRequestAsync<T> throws NotSupportedException from RequestDeSerializationProvider.Provide:

System.NotSupportedException: The BranchRequest is not supported by the RequestDeSerializationProvider.
   at SysML2.NET.Serializer.Json.PSM.RequestDeSerializationProvider.Provide(String typeName)
   at SysML2.NET.Serializer.Json.PSM.PsmDeSerializationDispatcher.ReadRequest(...)
   at SysML2.NET.Serializer.Json.DeSerializer.ReadFamilyObject[T](...)

Other malformed payloads surface differently: a missing @type gives a SerializationException, an @type of another request type than T gives a JsonException, and invalid JSON gives a JsonException / InvalidOperationException.

NotSupportedException usually signals "valid input the library cannot handle", so callers that map exceptions to responses (e.g. 400 for malformed input, 501 for unsupported features) misreport a client error. None of these exceptions is documented on IDeSerializer.

Expected: an unrecognised @type in an untrusted payload is reported as malformed input, consistently with the other cases — preferably a JsonException naming the unknown discriminator — and the exceptions are documented with <exception> tags on IDeSerializer. The same applies to the response family (ResponseDeSerializationProvider).

Steps to Reproduce

var deSerializer = new DeSerializer();
using var stream = new MemoryStream(Encoding.UTF8.GetBytes("""{ "@type": "BranchRequest", "name": "feature" }"""));
deSerializer.DeSerializeRequest<BranchRequest>(stream, SerializationModeKind.JSON, false);
// throws NotSupportedException instead of JsonException

System Configuration

• SysML2.NET version: 0.25.1 (SysML2.NET.Serializer.Json)
• Environment (Operating system, version and so on): Windows 11
• .NET Framework version: .NET 10
• Additional information: Mycelium Fabric currently also catches NotSupportedException when reading request bodies and answers 400.