[13.x] Add Batch::failureCallbacks for callback inspection

[miladev95]

Batch now exposes failureCallbacks() so stored batches can be inspected in the same way as PendingBatch.

PendingBatch already has failureCallbacks(), but Illuminate\Bus\Batch only exposed the boolean callback checks.
This change fills that gap and keeps the pending and persisted batch APIs aligned.

The new method returns the registered failure callbacks from the batch options, or an empty array when none are present. I also added coverage in the Bus batch state tests for both hasFailureCallbacks() and failureCallbacks().