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GH-26648: [C++] Optimize union equality comparison (#45384)



### Rationale for this change

#26648 proposes an optimization in checking sparse array equality by detecting contiguous runs, this PR implements that change

### What changes are included in this PR?

previously, sparse array comparison was checked one by one, in this change, contiguous runs are detected and compared by checking equality of current and previous child_ids

### Are these changes tested?

already covered by existing unit tests

### Are there any user-facing changes?

no

* GitHub Issue: #26648

Lead-authored-by: shawn <shawnmcneill10@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>