fix: Subworkflows do not work well with HostAsAgent (#3240)

Subworkflows run into issues with Checkpointing and the Chat Protocol:

* The concurrency rework made subtle changes in behaviour that introduced a hang when using subworkflows with ChatProtocol and streaming execution.
* The ResetAsync() implementation in WorkflowHostExecutor was improperly resetting the joinContext - this was happening on restore checkpoint _after_ the join context was attached when
* Subworkflows cannot be used as the start node when hosted AsAgent due to inability to treat Catch-All as a Chat Protocol
* Subworkflow ownership issue when used in non-concurrent mode after finishing a run

Also fixes:
* When ChatMessages are output by executors that are not agents, there is no corresponding AgentResponseUpdate/AgentResponse event

Breaking Changes
* [BREAKING CHANGE] It is possible to provide the wrong RunId when resuming from CheckpointInfo (even though the data already exists on CheckpointInfo)

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

@@ -29,6 +29,12 @@ public abstract class ChatProtocolExecutor : StatefulExecutor<List<ChatMessage>>
     private static readonly Func<List<ChatMessage>> s_initFunction = () => [];
     private readonly ChatRole? _stringMessageChatRole;
 
+    private static readonly StatefulExecutorOptions s_baseExecutorOptions = new()
+    {
+        AutoSendMessageHandlerResultObject = false,
+        AutoYieldOutputHandlerResultObject = false
+    };
+
     /// <summary>
     /// Initializes a new instance of the <see cref="ChatProtocolExecutor"/> class.
     /// </summary>
@@ -36,7 +42,7 @@ public abstract class ChatProtocolExecutor : StatefulExecutor<List<ChatMessage>>
     /// <param name="options">Optional configuration settings for the executor. If null, default options are used.</param>
     /// <param name="declareCrossRunShareable">Declare that this executor may be used simultaneously by multiple runs safely.</param>
     protected ChatProtocolExecutor(string id, ChatProtocolExecutorOptions? options = null, bool declareCrossRunShareable = false)
-        : base(id, () => [], declareCrossRunShareable: declareCrossRunShareable)
+        : base(id, () => [], s_baseExecutorOptions, declareCrossRunShareable)
     {
         this._stringMessageChatRole = options?.StringMessageChatRole;
     }