Batch: Fixes null ErrorMessage when promoting status from MultiStatus response

[NaluTripician]

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 null. 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 responseStatusCode and responseSubStatusCode from the first failing operation result, but the ErrorMessage passed to the TransactionalBatchResponse constructor was still responseMessage.ErrorMessage — the outer 207 MultiStatus message's error, which is always null.

// 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.

// 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.