Unable to use interface as event argument with Union-non-attributed interface

[dtivel]

Repro steps:

1. Clone this fork.
2. Switch to the repro1 branch.
3. Open the solution in Visual Studio.
4. Debug test Repro.

Result: The following exception is thrown:

MessagePack.FormatterNotRegisteredException
  HResult=0x80131500
  Message=MessagePackFormatterTests+Fruit is not registered in resolver: StreamJsonRpc.MessagePackFormatter+ResolverWrapper
  Source=MessagePack
  StackTrace:
   at MessagePack.FormatterResolverExtensions.Throw(Type t, IFormatterResolver resolver)

This is similar to #611; however, the differences are that in this bug the interface type (vs the concrete type) has a formatter and the Union attribute is not used. Plus, the exception is different.

I'd expect that the interface in the event definition would be sufficient hint for StreamJsonRpc to find the registered IFruit formatter.

public event EventHandler<IFruit>? OnFruit;