Could we provide a potentially larger InternalParquetRecordWriter.getDataSize

The following code currently has getDataSize as an estimated value. The Iceberg rolling file write operation relies on this method, which may result in writing files that are much smaller than expected.
```JAVA
/**
* @return the total size of data written to the file and buffered in memory
*/
public long getDataSize() {
return lastRowGroupEndPos + columnStore.getBufferedSize();
}
```

Could we provide a potentially larger getDataSize? I can't think of any downsides at the moment.

### Component(s)

_No response_

Locate InternalParquetRecordWriter.getDataSize and trace how Iceberg's rolling file write operation uses its value. Compare the reported size with actual written and buffered data, then define and test a larger, safe size estimate that prevents unexpectedly small files.

Written by the indexing model from the issue text.