[C++][Parquet] Replace remaining RapidJSON usage with simdjson #50779

Describe the enhancement requested

The Parquet module still contains a few remaining RapidJSON usages for JSON parsing, validation, and serialization under cpp/src/parquet.

Replace these remaining usages with the existing simdjson helpers. This also removes the remaining RapidJSON dependency from the Parquet test target and Meson build configuration.

Additional context

This is a follow-up task for GH-35460.

This migration removes the remaining RapidJSON usage under cpp/src/parquet while reusing the common simdjson abstractions introduced in previous migration PRs.

Parent Issue: #35460

Component(s)

C++, Parquet