Doesn't work with large batches

Messages are looked at individually as to whether they exceed the maximum, but a SendMessageBatchRequest might exceed 256kb in aggregate even if none of the individual messages do. It is somewhat confusing given this library appears to support batching.

Start by tracing the SendMessageBatchRequest handling path and inspect how the 256kb limit is checked for individual messages versus the aggregate request. Compare this with the library's stated batching support and determine the expected behavior for an oversized aggregate batch. Done means aggregate batch size handling is consistent with the supported behavior and covered by tests.

Written by the indexing model from the issue text.