MR: IcebergInputFormat drops identity partition constants

[thswlsqls]

Apache Iceberg version
main @ 9d7b2c5

Query engine
None — the bug is in the MapReduce/Hive input format (iceberg-mr).

Please describe the bug
IcebergInputFormat.IcebergRecordReader.openTask() (mr/src/main/java/org/apache/iceberg/mr/mapreduce/IcebergInputFormat.java line 310) builds the reader without idToConstant(...), so identity partition values recorded only in the manifest are never handed to the reader. Reading such a file returns null in the partition columns.

This is a regression from a2802c4 (#15333), which merged the Avro, ORC and Parquet branches into one ReadBuilder. Before that change all three passed constantsMap(task, IdentityPartitionConverters::convertConstant). The sibling reader touched by the same PR, data/src/main/java/org/apache/iceberg/data/GenericReader.openFile(), kept .idToConstant(partition).

Steps to reproduce

1. Create an identity-partitioned table.
2. Add a data file that does not physically contain the partition columns, with the partition values set on the manifest entry. TableMigrationUtil.listPartition(...) produces exactly this — it takes partition values from Hive metadata, not from the file — so any table imported with add_files or migrate is in this state.
3. Read the table through IcebergInputFormat.

Expected: the partition columns hold the manifest values. Actual: they are null.

Additional context
N/A