fix(dispatcher): throw on ambiguous handlers instead of silent .First()

Multiple handlers for the same request were previously resolved via .First(),
with ordering depending on Type.GetInterfaces() order (non-deterministic).
Now ScalarRequestInvoker/StreamRequestInvoker throw InvalidOperationException
listing the candidate handlers when more than one matches.

src/request.dispatcher/ScalarRequestInvoker.cs

@@ -43,7 +43,20 @@ internal sealed class ScalarRequestInvoker<TRequest, TResponse> : ScalarRequestI
 
 		Task<TResponse> Head(IScalarRequest<TResponse> r, CancellationToken ct)
 		{
-			return options.GetRequestHandlers<IScalarRequestHandler<TRequest, TResponse>>(serviceProvider).First().HandleAsync((TRequest)r, ct);
+			var handlers = options.GetRequestHandlers<IScalarRequestHandler<TRequest, TResponse>>(serviceProvider).ToArray();
+
+			if (handlers.Length is 0)
+			{
+				throw new InvalidOperationException($"No handler is registered for request '{typeof(TRequest).FullName}'.");
+			}
+
+			if (handlers.Length > 1)
+			{
+				var candidates = string.Join(", ", handlers.Select(handler => handler.GetType().FullName));
+				throw new InvalidOperationException($"Multiple handlers are registered for request '{typeof(TRequest).FullName}'. Ambiguous handlers: {candidates}");
+			}
+
+			return handlers[0].HandleAsync((TRequest)r, ct);
 		}
 	}
 }