.NET: Expand Workflow Unit Test Coverage (#5390)

* refactor: remove dead code

* refactor: remove ignore YieldsMessageAttribute

- the correct one to use is YieldsOutputAttribute
- fixes a comment that mistakenly refers to `.YieldsMessage()` which does not exist.

* fix: ChatForwardingExecutor does not use correct role for string messages

- make ChatForwardingExecutor use its configured role for string messages rather than always use ChatRole.User
- add ChatForwardingExecutor tests

* fixup: remove unused attribute

* test: Add tests for failure when .AsAgent used on a non-ChatProtocol workflow

* test: Add FunctionExecutor tests

- also fixes Send and YieldOutput type registration for synchronous output-returning delegates

* test: Suppress CodeCoverage for obsolete names

* fix: Re-add Obsolete attributes

- avoid hard-breaking change
- properly notify users that these attributes get ignored

dotnet/src/Microsoft.Agents.AI.Workflows/ChatForwardingExecutor.cs

@@ -47,7 +47,7 @@ public sealed class ChatForwardingExecutor(string id, ChatForwardingExecutorOpti
             if (this._stringMessageChatRole.HasValue)
             {
                 routeBuilder = routeBuilder.AddHandler<string>(
-                    (message, context) => context.SendMessageAsync(new ChatMessage(ChatRole.User, message)));
+                    (message, context) => context.SendMessageAsync(new ChatMessage(this._stringMessageChatRole.Value, message)));
             }
 
             routeBuilder.AddHandler<ChatMessage>(ForwardMessageAsync)