otel-arrow-rust batching logic panic

[jmacd]

otap_batch_processor.rs has an ignored test:

    // Test constants for batching smoke tests
    #[test]
    #[ignore = "upstream-batching-bug"]
    // NOTE: Validates cross-signal partitioning and expected row totals.
    fn test_make_output_batches_partitions_and_splits() {

I do not believe we have recorded the underlying bug. Indeed, when you unignore this test there is a panic in the underlying otel-arrow-rust groups.rs.