[codex] Add choose decimal dispatch regression test

[zanmato1984]

Summary

• add a regression test for mixed decimal dispatch in choose
• show that DispatchBest() currently leaves {decimal128(3, 2), decimal128(4, 3)} unnormalized

Why

The new test demonstrates that choose does not perform decimal common-type normalization in dispatch, unlike related functions such as if_else, case_when, and coalesce.

Validation

• built arrow-compute-scalar-if-else-test in a fresh local CMake build tree
• TestChooseKernel.DispatchBest: fails with the new assertion
• TestChoose.Decimal: still passes

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview