Batch: Fixes null ErrorMessage when promoting status from MultiStatus response (#5650)

## Summary

Fixes #5649

When a transactional batch response returns **207 MultiStatus** and the
SDK promotes the status code from a failing operation (e.g., 409
Conflict), the \TransactionalBatchResponse.ErrorMessage\ was always \
ull\. This is because only \StatusCode\ and \SubStatusCode\ were
promoted from the failing operation — the \ErrorMessage\ remained
sourced from the outer 207 response, which has no error message.

## Root Cause

In \TransactionalBatchResponse.PopulateFromContentAsync()\, the status
promotion logic (lines 365–379) correctly promotes \
esponseStatusCode\ and \
esponseSubStatusCode\ from the first failing operation result, but the
\ErrorMessage\ passed to the \TransactionalBatchResponse\ constructor
was still \
esponseMessage.ErrorMessage\ — the outer 207 MultiStatus message's
error, which is always `null`.

\\\csharp
// Before (bug):
TransactionalBatchResponse response = new TransactionalBatchResponse(
    responseStatusCode,              // 409 - promoted from operation ✅
    responseSubStatusCode,           // promoted from operation ✅
    responseMessage.ErrorMessage,    // null from outer 207 response ❌
    ...);
\\\

## Fix

When promoting the status code from a failing operation, also read the
error message from the failing operation's \ResourceStream\ (which
contains the server's error body for that specific operation) and use it
as the promoted \ErrorMessage\.

\\\csharp
// After (fix):
if (result.ResourceStream != null)
{
    using (StreamReader reader = new StreamReader(
        result.ResourceStream, encoding: Encoding.UTF8,
        detectEncodingFromByteOrderMarks: true,
        bufferSize: 1024, leaveOpen: true))
    {
        responseErrorMessage = reader.ReadToEnd();
result.ResourceStream.Position = 0; // Reset so stream is still usable
    }
}
\\\

The stream position is reset after reading so the \ResourceStream\
remains available to consumers who access individual operation results.

## Testing

Added \BatchResponseDeserializationPromotesErrorMessageAsync\ test that:
1. Creates a 409 Conflict operation result with a \ResourceStream\
containing an error body
2. Creates a second operation result with 424 FailedDependency (as the
server would)
3. Wraps them in a 207 MultiStatus response
4. Calls \FromResponseMessageAsync\ with
\shouldPromoteOperationStatus=true\
5. Asserts \StatusCode\ is promoted to 409
6. **Asserts \ErrorMessage\ is promoted from the operation's
ResourceStream** (not null)

All 103 existing batch-related unit tests continue to pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: NaluTripician

Microsoft.Azure.Cosmos/src/Batch/TransactionalBatchResponse.cs

@@ -361,6 +361,7 @@ private static async Task<TransactionalBatchResponse> PopulateFromContentAsync(
 
             HttpStatusCode responseStatusCode = responseMessage.StatusCode;
             SubStatusCodes responseSubStatusCode = responseMessage.Headers.SubStatusCode;
+            string responseErrorMessage = responseMessage.ErrorMessage;
 
             // Promote the operation error status as the Batch response error status if we have a MultiStatus response
             // to provide users with status codes they are used to.
@@ -373,6 +374,16 @@ private static async Task<TransactionalBatchResponse> PopulateFromContentAsync(
                     {
                         responseStatusCode = result.StatusCode;
                         responseSubStatusCode = result.SubStatusCode;
+
+                        if (result.ResourceStream != null)
+                        {
+                            using (StreamReader reader = new StreamReader(result.ResourceStream, encoding: System.Text.Encoding.UTF8, detectEncodingFromByteOrderMarks: true, bufferSize: 1024, leaveOpen: true))
+                            {
+                                responseErrorMessage = reader.ReadToEnd();
+                                result.ResourceStream.Position = 0;
+                            }
+                        }
+
                         break;
                     }
                 }
@@ -381,7 +392,7 @@ private static async Task<TransactionalBatchResponse> PopulateFromContentAsync(
             TransactionalBatchResponse response = new TransactionalBatchResponse(
                 responseStatusCode,
                 responseSubStatusCode,
-                responseMessage.ErrorMessage,
+                responseErrorMessage,
                 responseMessage.Headers,
                 trace,
                 serverRequest.Operations,

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/Batch/BatchSchemaTests.cs

@@ -177,7 +177,59 @@ public async Task BatchResponseDeserializationAsync()
             Assert.IsTrue(comparer.Equals(results[1], batchResponse[1]));
         }
 
-        private class ItemBatchOperationEqualityComparer : IEqualityComparer<ItemBatchOperation>
+        [TestMethod]
+        [Owner("nalutripician")]
+        public async Task BatchResponseDeserializationPromotesErrorMessageAsync()
+        {
+            string expectedErrorMessage = "{\"Errors\":[\"Resource with specified id or name already exists.\"]}";
+            byte[] errorBody = System.Text.Encoding.UTF8.GetBytes(expectedErrorMessage);
+
+            using CosmosClient cosmosClient = MockCosmosUtil.CreateMockCosmosClient();
+            ContainerInternal containerCore = (ContainerInlineCore)cosmosClient.GetDatabase("db").GetContainer("cont");
+            List<TransactionalBatchOperationResult> results = new List<TransactionalBatchOperationResult>
+            {
+                new TransactionalBatchOperationResult(HttpStatusCode.Conflict)
+                {
+                    ResourceStream = new CloneableStream(
+                        internalStream: new MemoryStream(errorBody, index: 0, count: errorBody.Length, writable: false, publiclyVisible: true),
+                        allowUnsafeDataAccess: true),
+                },
+                new TransactionalBatchOperationResult(HttpStatusCode.FailedDependency)
+            };
+
+            MemoryStream responseContent = await new BatchResponsePayloadWriter(results).GeneratePayloadAsync();
+
+            SinglePartitionKeyServerBatchRequest batchRequest = await SinglePartitionKeyServerBatchRequest.CreateAsync(
+                partitionKey: Cosmos.PartitionKey.None,
+                operations: new ArraySegment<ItemBatchOperation>(
+                    new ItemBatchOperation[]
+                    {
+                        new ItemBatchOperation(OperationType.Create, operationIndex: 0, id: "someId", containerCore: containerCore),
+                        new ItemBatchOperation(OperationType.Create, operationIndex: 1, id: "someId2", containerCore: containerCore)
+                    }),
+                serializerCore: MockCosmosUtil.Serializer,
+                trace: NoOpTrace.Singleton,
+                cancellationToken: CancellationToken.None);
+
+            ResponseMessage response = new ResponseMessage((HttpStatusCode)StatusCodes.MultiStatus) { Content = responseContent };
+            response.Headers.Session = Guid.NewGuid().ToString();
+            response.Headers.ActivityId = Guid.NewGuid().ToString();
+
+            TransactionalBatchResponse batchResponse = await TransactionalBatchResponse.FromResponseMessageAsync(
+                response,
+                batchRequest,
+                MockCosmosUtil.Serializer,
+                true,
+                NoOpTrace.Singleton,
+                CancellationToken.None);
+
+            Assert.IsNotNull(batchResponse);
+            Assert.AreEqual(HttpStatusCode.Conflict, batchResponse.StatusCode);
+            Assert.IsNotNull(batchResponse.ErrorMessage, "ErrorMessage should be promoted from the failing operation's ResourceStream");
+            Assert.AreEqual(expectedErrorMessage, batchResponse.ErrorMessage);
+        }
+
+        private class ItemBatchOperationEqualityComparer: IEqualityComparer<ItemBatchOperation>
         {
             public bool Equals(ItemBatchOperation x, ItemBatchOperation y)
             {