Add AsChatClient for OpenAI's AssistantClient (#5852)

- Adds ChatOptions/ChatCompletion/StreamingChatCompletionUpdate.ChatThreadId.
- Adds an AsChatClient extension method for creating an IChatClient from an AssistantClient.
- Updates FunctionInvokingChatClient to have KeepFunctionCallingMessages
  be false by default, and to handle ChatThreadId.
- Fixes handling of ChatCompletion.Usage in ToChatCompletion{Async}.

src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatCompletion.cs

@@ -61,6 +61,16 @@ public ChatMessage Message
     /// <summary>Gets or sets the ID of the chat completion.</summary>
     public string? CompletionId { get; set; }
 
+    /// <summary>Gets or sets the chat thread ID associated with this chat completion.</summary>
+    /// <remarks>
+    /// Some <see cref="IChatClient"/> implementations are capable of storing the state for a chat thread, such that
+    /// the input messages supplied to <see cref="IChatClient.CompleteAsync"/> need only be the additional messages beyond
+    /// what's already stored. If this property is non-<see langword="null"/>, it represents an identifier for that state,
+    /// and it should be used in a subsequent <see cref="ChatOptions.ChatThreadId"/> instead of supplying the same messages
+    /// (and this <see cref="ChatCompletion"/>'s message) as part of the <c>chatMessages</c> parameter.
+    /// </remarks>
+    public string? ChatThreadId { get; set; }
+
     /// <summary>Gets or sets the model ID used in the creation of the chat completion.</summary>
     public string? ModelId { get; set; }
 
@@ -133,6 +143,7 @@ public StreamingChatCompletionUpdate[] ToStreamingChatCompletionUpdates()
             ChatMessage choice = Choices[choiceIndex];
             updates[choiceIndex] = new StreamingChatCompletionUpdate
             {
+                ChatThreadId = ChatThreadId,
                 ChoiceIndex = choiceIndex,
 
                 AdditionalProperties = choice.AdditionalProperties,

src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatMessage.cs

@@ -39,6 +39,22 @@ public ChatMessage(
         _contents = Throw.IfNull(contents);
     }
 
+    /// <summary>Clones the <see cref="ChatMessage"/> to a new <see cref="ChatMessage"/> instance.</summary>
+    /// <returns>A shallow clone of the original message object.</returns>
+    /// <remarks>
+    /// This is a shallow clone. The returned instance is different from the original, but all properties
+    /// refer to the same objects as the original.
+    /// </remarks>
+    public ChatMessage Clone() =>
+        new()
+        {
+            AdditionalProperties = AdditionalProperties,
+            _authorName = _authorName,
+            _contents = _contents,
+            RawRepresentation = RawRepresentation,
+            Role = Role,
+        };
+
     /// <summary>Gets or sets the name of the author of the message.</summary>
     public string? AuthorName
     {

src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatOptions.cs

@@ -9,6 +9,9 @@ namespace Microsoft.Extensions.AI;
 /// <summary>Represents the options for a chat request.</summary>
 public class ChatOptions
 {
+    /// <summary>Gets or sets an optional identifier used to associate a request with an existing chat thread.</summary>
+    public string? ChatThreadId { get; set; }
+
     /// <summary>Gets or sets the temperature for generating chat responses.</summary>
     public float? Temperature { get; set; }
 
@@ -72,6 +75,7 @@ public virtual ChatOptions Clone()
     {
         ChatOptions options = new()
         {
+            ChatThreadId = ChatThreadId,
             Temperature = Temperature,
             MaxOutputTokens = MaxOutputTokens,
             TopP = TopP,

src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/StreamingChatCompletionUpdate.cs

@@ -102,6 +102,16 @@ public IList<AIContent> Contents
     /// <summary>Gets or sets the ID of the completion of which this update is a part.</summary>
     public string? CompletionId { get; set; }
 
+    /// <summary>Gets or sets the chat thread ID associated with the chat completion of which this update is a part.</summary>
+    /// <remarks>
+    /// Some <see cref="IChatClient"/> implementations are capable of storing the state for a chat thread, such that
+    /// the input messages supplied to <see cref="IChatClient.CompleteStreamingAsync"/> need only be the additional messages beyond
+    /// what's already stored. If this property is non-<see langword="null"/>, it represents an identifier for that state,
+    /// and it should be used in a subsequent <see cref="ChatOptions.ChatThreadId"/> instead of supplying the same messages
+    /// (and this streaming message) as part of the <c>chatMessages</c> parameter.
+    /// </remarks>
+    public string? ChatThreadId { get; set; }
+
     /// <summary>Gets or sets a timestamp for the completion update.</summary>
     public DateTimeOffset? CreatedAt { get; set; }
 

src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/StreamingChatCompletionUpdateExtensions.cs

@@ -13,6 +13,7 @@
 
 #pragma warning disable S109 // Magic numbers should not be used
 #pragma warning disable S127 // "for" loop stop conditions should be invariant
+#pragma warning disable S1121 // Assignments should not be made from within sub-expressions
 
 namespace Microsoft.Extensions.AI;
 
@@ -103,7 +104,21 @@ private static void ProcessUpdate(StreamingChatCompletionUpdate update, Dictiona
         }
 #endif
 
-        ((List<AIContent>)message.Contents).AddRange(update.Contents);
+        // Incorporate all content from the update into the completion.
+        foreach (var content in update.Contents)
+        {
+            switch (content)
+            {
+                // Usage content is treated specially and propagated to the completion's Usage.
+                case UsageContent usage:
+                    (completion.Usage ??= new()).Add(usage.Details);
+                    break;
+
+                default:
+                    message.Contents.Add(content);
+                    break;
+            }
+        }
 
         message.AuthorName ??= update.AuthorName;
         if (update.Role is ChatRole role && message.Role == default)
@@ -178,20 +193,6 @@ static void AddMessage(ChatCompletion completion, bool coalesceContent, KeyValue
             }
 
             completion.Choices.Add(entry.Value);
-
-            if (completion.Usage is null)
-            {
-                foreach (var content in entry.Value.Contents)
-                {
-                    if (content is UsageContent c)
-                    {
-                        completion.Usage = c.Details;
-                        entry.Value.Contents = entry.Value.Contents.ToList();
-                        _ = entry.Value.Contents.Remove(c);
-                        break;
-                    }
-                }
-            }
         }
     }