Project only accessed struct leaves in Parquet row filter pushdown (#20854)

## Which issue does this PR close?

- Related #20822
- Closes #20603

## Rationale for this change

This PR refines how the `FilterCandidateBuilder` projects struct columns
during Parquet row filter pushdown.

Previously, a filter like `s['value'] > 10` would cause the reader to
decode all leaf columns of a struct `s`, because `PushdownChecker` only
tracked the root column index and expanded it to every leaf. This wastes
I/O and decode time on fields the filter never touches

Now, the builder resolves only the matching Parquet leaf columns. It
does this by building a pruned filter schema that reflects exactly what
the Parquet reader produces when projecting a subset of struct leaves,
ensuring the expression evaluates against the correct types

Author: friendlymatthew