MR: IcebergInputFormat drops identity partition constants

**Apache Iceberg version**
main @ 9d7b2c52f

**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 a2802c44c (#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

Start in mr/src/main/java/org/apache/iceberg/mr/mapreduce/IcebergInputFormat.java at IcebergRecordReader.openTask(), then compare it with data/src/main/java/org/apache/iceberg/data/GenericReader.openFile(). Reproduce the identity-partitioned case described using manifest-only partition values and verify that IcebergInputFormat returns those values in the partition columns instead of null.

Written by the indexing model from the issue text.