fix: Initialize ChatResponseUpdate.MessageId with ResponseId

Fixes #15327

Author: amanda-tarafa

apis/Google.Cloud.VertexAI.Extensions/Google.Cloud.VertexAI.Extensions.Tests/BuildIChatClientTest.cs

@@ -1757,6 +1757,11 @@ public async Task IChatClient_GetStreamingResponseAsync_BasicStreaming()
 
         string[] texts = [.. updates.Select(u => ((TextContent) u.Contents[0]).Text)];
         Assert.Equal(["Hello", " world", "!"], texts);
+        string?[] messageIds = [.. updates.Select(u => u.MessageId)];
+        string?[] responseIds = [.. updates.Select(u => u.ResponseId)];
+        string[] expectedIds = ["response-1", "response-2", "response-3"];
+        Assert.Equal(expectedIds, messageIds);
+        Assert.Equal(expectedIds, responseIds);
     }
 
     [Fact]

apis/Google.Cloud.VertexAI.Extensions/Google.Cloud.VertexAI.Extensions/PredictionServiceChatClient.cs

@@ -104,6 +104,7 @@ public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(IEnu
                 ModelId = !string.IsNullOrEmpty(generateResult.ModelVersion) ? generateResult.ModelVersion : request.Model,
                 RawRepresentation = generateResult,
                 ResponseId = generateResult.ResponseId,
+                MessageId = generateResult.ResponseId,
             };
 
             // Populate the response update contents.