Fix invalid cast issue in omnisharp-roslyn by fixing LspHandlerDescriptor .ctor

The issue (apparently) is that LspHandlerDescriptor is constructed for
DelegatingRequest that is registered in LanguageServerHost.cs (in
omnisharp-roslyn) via .OnRequest().

The constructor attempts to resolve RPC return type based on what
generic type is set on IRequest<T> that inherits DelegatingRequest.
However DelegatingRequest implements both IRequest and
IRequest<TResponse> and .LINQ query selects IRequest<MediatR.Unit> type
as the return value -- while it should be IRequest<JToken>.

Fixed by ordering implemented IRequest<> interfaces by generic param
type and taking the first that is not MediatR.Unit -- only then use the
MediatR.Unit version.

Author: razzmatazz

src/Protocol/Shared/LspHandlerDescriptor.cs

@@ -69,8 +69,11 @@ public LspHandlerDescriptor(
             CapabilityType = capabilityType;
 
             Response = @params?.GetInterfaces()
-                               .FirstOrDefault(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(IRequest<>))
-                              ?.GetGenericArguments()[0] ?? typeDescriptor?.ResponseType ?? typeof(Unit);
+                               .Where(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(IRequest<>))
+                               .Select(x => x.GetGenericArguments()[0])
+                               .OrderBy(x => x == typeof(Unit))
+                               .FirstOrDefault()
+                              ?? typeDescriptor?.ResponseType ?? typeof(Unit);
 
             // If multiple are implemented this behavior is unknown
             CanBeResolvedHandlerType = handler.GetType().GetTypeInfo()