[C++][Compute] Add decimal dispatch regression tests

[zanmato1984]

Adds focused regression/validation cases for decimal dispatch behavior around match constraints and expression binding.

What changed:

• Adds expression bind cases for decimal if_else, coalesce, and choose.
• Adds dispatch-level decimal cases for coalesce and choose.

Baseline:

• This PR targets codex/if-else-match-constraint inside the fork repo so the diff only contains the test cases, not the existing match-constraint implementation.

Validation:

• git diff --check passes.
• .build-choose-test/release/arrow-compute-expression-test --gtest_filter=Expression.BindWithImplicitCastsForIfElseOnDecimal:Expression.BindWithImplicitCastsForCoalesceOnDecimal:Expression.BindWithImplicitCastsForChooseOnDecimal currently has 1 pass and 2 expected failures: coalesce and choose expression binding do not insert the decimal cast.
• .build-choose-test/release/arrow-compute-scalar-if-else-test --gtest_filter=TestCoalesce.DispatchBest:TestChooseKernel.DispatchBest currently has 1 pass and 1 expected failure: choose DispatchBest does not normalize decimal value args yet.

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview