refactor: Reduce reflection calls when using HandlerInvoker (#5241)

Changes `HandlerInvoker` to avoid reflecting for every message when handling arity=2 `IHandler<,>` instances, by moving it out of the final generated delegate.

dotnet/src/Microsoft.AutoGen/Core/BaseAgent.cs

@@ -70,9 +70,10 @@ public abstract class BaseAgent : IAgent, IHostableAgent
         Dictionary<Type, HandlerInvoker> invokers = new();
         foreach (Type interface_ in candidateInterfaces)
         {
-            MethodInfo? maybeHandle = interface_.GetMethod(nameof(IHandle<object>.HandleAsync), BindingFlags.Instance | BindingFlags.Public);
+            MethodInfo handleAsync = interface_.GetMethod(nameof(IHandle<object>.HandleAsync), BindingFlags.Instance | BindingFlags.Public)
+                                     ?? throw new InvalidOperationException($"No handler method found for interface {interface_.FullName}");
 
-            HandlerInvoker invoker = new(maybeHandle ?? throw new InvalidOperationException($"No handler method found for interface {interface_.FullName}"), this);
+            HandlerInvoker invoker = new(handleAsync, this);
             invokers.Add(interface_.GetGenericArguments()[0], invoker);
         }
 

dotnet/src/Microsoft.AutoGen/Core/HandlerInvoker.cs

@@ -42,19 +42,17 @@ public class HandlerInvoker
                 return null;
             };
         }
-        else if (
-            methodInfo.ReturnType.GetGenericTypeDefinition() == typeof(ValueTask<>)
-            )
+        else if (methodInfo.ReturnType.GetGenericTypeDefinition() == typeof(ValueTask<>))
         {
+            MethodInfo typeEraseAwait = typeof(HandlerInvoker)
+                    .GetMethod(nameof(TypeEraseAwait), BindingFlags.NonPublic | BindingFlags.Static)!
+                    .MakeGenericMethod(methodInfo.ReturnType.GetGenericArguments()[0]);
+
             getResultAsync = async
             (object? message, MessageContext messageContext) =>
             {
                 object valueTask = invocation(message, messageContext)!;
-
-                object? typelessValueTask = typeof(HandlerInvoker)
-                    .GetMethod(nameof(TypeEraseAwait), BindingFlags.NonPublic | BindingFlags.Static)!
-                    .MakeGenericMethod(methodInfo.ReturnType.GetGenericArguments()[0])
-                    .Invoke(null, new object[] { valueTask });
+                object? typelessValueTask = typeEraseAwait.Invoke(null, new object[] { valueTask });
 
                 Debug.Assert(typelessValueTask is ValueTask<object?>);