Initial Default values gets ignored for Struct columns

### Apache Iceberg version

All

### Query engine

All

### Please describe the bug 🐞

The initialDefault value in case of a Struct column is always null, but its fields can have default value, The Iceberg layer allows to set & store initialDefault values for the primitive fields of a Struct type. But doesn't honor them while reading.

For Paquet:
https://github.com/apache/iceberg/blob/c02d4676f38b834d6c672bb53abd34b07ebfddb9/parquet/src/main/java/org/apache/iceberg/data/parquet/BaseParquetReaders.java#L289-L292

This code block checks whether the initialDefault is not null, for Struct type it will always be `null` but its fields can have default values.

It should check for the nested fields in case of struct or not allow having initialDefault values for sub fields of Struct type:

A sample screenshot:

The field Initial Default is `null` but the nested fields like `x` have an initial default of `100`, but this gets ignored

### Willingness to contribute

- [ ] I can contribute a fix for this bug independently
- [ ] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

Start in parquet/src/main/java/org/apache/iceberg/data/parquet/BaseParquetReaders.java around lines 289-292, where the reader checks the struct column's initialDefault. Trace how nested primitive defaults are represented and read for a struct; the issue is done when those nested defaults are honored, or the unsupported combination is rejected consistently.

Written by the indexing model from the issue text.