[Parquet] Add metadata heap size test that tracks allocations

See relevant comments here: https://github.com/apache/arrow-rs/pull/8898#issuecomment-3562631348

There are tests that use hard-coded expected memory sizes (eg [here](https://github.com/apache/arrow-rs/blob/1c90efe717c1a2f99a0a97d6e9c93cd913eaca3c/parquet/src/file/metadata/mod.rs#L1815)), but these expected sizes could become incorrect if the metadata layout changes.

Start in parquet/src/file/metadata/mod.rs around the hard-coded metadata memory-size test at the linked location, then read the referenced comments on pull request 8898. Add a metadata heap-size test that tracks allocations and remains valid when the metadata layout changes; existing metadata tests should continue to pass.

Written by the indexing model from the issue text.