[codex] Refactor if_else decimal dispatch with match constraint

[zanmato1984]

Summary

• add a decimal match constraint to if_else kernels
• make mixed decimal inputs miss DispatchExact and fall back to DispatchBest normalization
• add regression coverage for the new exact-match behavior

Why

This refactors part of if_else dispatch so that the exact-match acceptance rule for decimal kernels is expressed at the kernel signature layer rather than only implied by function-level dispatch behavior.

Validation

• built arrow-compute-scalar-if-else-test
• TestIfElseKernel.IfElseDispatchBest passes