.Net: Add support for 3rd party thread storage and thread serialization (#203)

* Add thread storage and serialization POC

* Switch to using JsonElement and add unit tests

* Add additional unit tests.

* Exclude private debugger properties from CodeCoverage.

* Rename IChatMessagesStorable to IChatMessageStore

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Improve xml doc.

* Update the message storing thread to always use external store for both local and remote storage.

* Remove threadid from the IChatMessageStore interface, since the store should own the thread id itself, if it requires one.

* Switch GetMessages to IEnumerable

* Address pr comments.

* Make jsonserializer options default consistent on DeserializeThreadAsync

* Move message storing thread functionality into AgentThread and simplify AgentThread behavior.

* Remove embedding generation from VectorStore chat history sample.

* Remove unecessary code and fix formatting.

* Make GetNewThread and DeserializeThread virtual with default implementations.
Remove unsued json utilities.

* Fix formatting

* Remove problem test.

* Add more unit tests

* Remove unused using clause.

* Address pr feedback.

* Address PR comments.

* Make InMemory store internal

* Switch InMemoryChatMessageStore to implement IList instead of inheriting from List.

* Rename store deserialize param.

* Update serialization based on PR comments.

* Remove confusing comment.

* Address Deserialization PR comments in the same way as Serialization

* Add State to IChatMessageStore Serialize and Deserialize names.
Make Thread Deserialize internal.
Make AgentThread type switching fobidden.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Chris <66376200+crickman@users.noreply.github.com>

dotnet/samples/HelloHttpApi/HelloHttpApi.ApiService/ChatClientAgentActor.cs

@@ -14,7 +14,7 @@ internal sealed class ChatClientAgentActor(
     ILogger<ChatClientAgentActor> logger) : IActor
 {
     private string? _etag;
-    private ChatClientAgentThread? _thread;
+    private AgentThread? _thread;
 
     public ValueTask DisposeAsync() => default;
 
@@ -34,11 +34,11 @@ internal sealed class ChatClientAgentActor(
             if (threadResult.Value is { } threadJson)
             {
                 // Deserialize the thread state if it exist
-                this._thread = threadJson.Deserialize(ChatClientAgentActorJsonContext.Default.ChatClientAgentThread);
+                this._thread = await agent.DeserializeThreadAsync(threadJson, cancellationToken: cancellationToken).ConfigureAwait(false);
             }
         }
 
-        this._thread ??= agent.GetNewThread() as ChatClientAgentThread ?? throw new InvalidOperationException("The agent did not provide a valid thread instance.");
+        this._thread ??= agent.GetNewThread();
         Log.ThreadStateRestored(logger, context.ActorId.ToString(), response.Results[0] is GetValueResult { Value: not null });
 
         while (!cancellationToken.IsCancellationRequested)

dotnet/samples/HelloHttpApi/HelloHttpApi.ApiService/ChatClientAgentActorJsonContext.cs

@@ -2,7 +2,6 @@
 
 using System.Text.Json;
 using System.Text.Json.Serialization;
-using Microsoft.Extensions.AI.Agents;
 
 namespace HelloHttpApi.ApiService;
 
@@ -14,6 +13,5 @@ namespace HelloHttpApi.ApiService;
     UseStringEnumConverter = true,
     DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
     WriteIndented = false)]
-[JsonSerializable(typeof(ChatClientAgentThread))]
 [JsonSerializable(typeof(ChatClientAgentRunRequest))]
 internal sealed partial class ChatClientAgentActorJsonContext : JsonSerializerContext;