fix: Update WorkflowHostAgent post Checkpointing implementation (#1498)

WorkflowHostAgent was initially implemented before Checkpointing was available. This meant that in order to support resuming, the WorkflowHostAgent needed to keep the runs around, which broke it when stricter rules about concurrent sharing of workflows during execution were introduced.

This change updates the hosting logic to release the underlying StreamingRun when the RunStreamingAsync or RunAsync are invoked, in favour of keeping the checkpointing information in the WorkflowThread to enable resumption.

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

@@ -1,11 +1,11 @@
 // Copyright (c) Microsoft. All rights reserved.
 
-using System;
 using System.Collections.Generic;
 using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;
+using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Agents.AI.Workflows;
 
@@ -18,22 +18,22 @@ internal sealed class WorkflowMessageStore : ChatMessageStore
     {
     }
 
-    public WorkflowMessageStore(JsonElement serializedStoreState, JsonSerializerOptions? jsonSerializerOptions = null)
+    public WorkflowMessageStore(StoreState state)
     {
-        if (serializedStoreState.ValueKind is not JsonValueKind.Object)
-        {
-            throw new ArgumentException("The provided JsonElement must be a json object", nameof(serializedStoreState));
-        }
+        this.ImportStoreState(Throw.IfNull(state));
+    }
 
-        StoreState? state =
-            serializedStoreState.Deserialize(
-                AgentAbstractionsJsonUtilities.DefaultOptions.GetTypeInfo(typeof(StoreState))) as StoreState;
+    private void ImportStoreState(StoreState state, bool clearMessages = false)
+    {
+        if (clearMessages)
+        {
+            this._chatMessages.Clear();
+        }
 
         if (state?.Messages is not null)
         {
             this._chatMessages.AddRange(state.Messages);
         }
-
         this._bookmark = state?.Bookmark ?? 0;
     }
 
@@ -66,13 +66,11 @@ internal sealed class WorkflowMessageStore : ChatMessageStore
 
     public override JsonElement Serialize(JsonSerializerOptions? jsonSerializerOptions = null)
     {
-        StoreState state = new()
-        {
-            Bookmark = this._bookmark,
-            Messages = this._chatMessages,
-        };
+        StoreState state = this.ExportStoreState();
 
         return JsonSerializer.SerializeToElement(state,
             WorkflowsJsonUtilities.DefaultOptions.GetTypeInfo(typeof(StoreState)));
     }
+
+    internal StoreState ExportStoreState() => new() { Bookmark = this._bookmark, Messages = this._chatMessages };
 }