fix: overflow error when concatenating more than 128 regular arrays

[ikrommyd]

Now actually finalize #3310

Instantiate prototype in the same way we do for tags_cls.
Without it, we get

  File "/Users/nmangane/scikit-hep-dev-3/.pixi/envs/default/lib/python3.13/site-packages/awkward/operations/ak_concatenate.py", line 262, in action
    prototype[start : start + size] = tag
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
OverflowError: Python integer 128 out of bounds for int8

This error occurred while calling

    ak.concatenate(
        [<Array [[False], [False], ..., [False]] type='20 * [1 * bool, parame...
        axis = 1
    )

[ianna]

@ikrommyd - Looks good! Thanks for providing the fix so quickly! This is the right solution since all the kernels involved have specialization for int64 and both UnionArray functions regular_index and simplified allow Index64 as tags. It would help if we add a test where simplified fails to reduce to no more than 128 variants. Please see the comments. Thanks!

# Note: to help merge more than 128 arrays, tags *can* have type ak.index.Index64.
# This is only supported when index is also Index64,
# and all indexed contents are also Index64.
# We still require that this reduces to no more than 128 variants.

[ianna]

@ikrommyd - Thanks for fixing it!