Update ToChatResponse{Async} to also factor in AuthorName (#6910)

* Update ToChatResponse{Async} to also factor in AuthorName

Originally we were only factoring in Role. Then we also augmented it to factor in MessageId, since updates from different messages are by definition not part of the same message. Now it also makes sense to factor in AuthorName, as different text from different speakers / agents are also by definition not part of the same message.

* Fix useAsync inversion in tests

Author: stephentoub

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

@@ -306,26 +306,19 @@ private static void FinalizeResponse(ChatResponse response)
     private static void ProcessUpdate(ChatResponseUpdate update, ChatResponse response)
     {
         // If there is no message created yet, or if the last update we saw had a different
-        // message ID or role than the newest update, create a new message.
-        ChatMessage message;
-        var isNewMessage = false;
-        if (response.Messages.Count == 0)
-        {
-            isNewMessage = true;
-        }
-        else if (update.MessageId is { Length: > 0 } updateMessageId
-            && response.Messages[response.Messages.Count - 1].MessageId is string lastMessageId
-            && updateMessageId != lastMessageId)
-        {
-            isNewMessage = true;
-        }
-        else if (update.Role is { } updateRole
-            && response.Messages[response.Messages.Count - 1].Role is { } lastRole
-            && updateRole != lastRole)
+        // identifying parts, create a new message.
+        bool isNewMessage = true;
+        if (response.Messages.Count != 0)
         {
-            isNewMessage = true;
+            var lastMessage = response.Messages[response.Messages.Count - 1];
+            isNewMessage =
+                NotEmptyOrEqual(update.AuthorName, lastMessage.AuthorName) ||
+                NotEmptyOrEqual(update.MessageId, lastMessage.MessageId) ||
+                NotNullOrEqual(update.Role, lastMessage.Role);
         }
 
+        // Get the message to target, either a new one or the last ones.
+        ChatMessage message;
         if (isNewMessage)
         {
             message = new(ChatRole.Assistant, []);
@@ -418,4 +411,12 @@ private static void ProcessUpdate(ChatResponseUpdate update, ChatResponse respon
             }
         }
     }
+
+    /// <summary>Gets whether both strings are not null/empty and not the same as each other.</summary>
+    private static bool NotEmptyOrEqual(string? s1, string? s2) =>
+        s1 is { Length: > 0 } str1 && s2 is { Length: > 0 } str2 && str1 != str2;
+
+    /// <summary>Gets whether two roles are not null and not the same as each other.</summary>
+    private static bool NotNullOrEqual(ChatRole? r1, ChatRole? r2) =>
+        r1.HasValue && r2.HasValue && r1.Value != r2.Value;
 }