CAS: EXPORT PARTITION from a CAS source is rejected with SUPPORT_IS_DISABLED

[DimensionWieldr]

Describe the bug
ALTER TABLE … EXPORT PARTITION against a MergeTree source on a CAS disk is refused with Code: 344 SUPPORT_IS_DISABLED. The CAS partition-command allowlist in MergeTreeData::checkAlterPartitionIsPossible does not include EXPORT_PARTITION.

To Reproduce
Source table on storage_policy = 'cas_policy', IcebergS3 destination, then:

ALTER TABLE mt_cas_export
    (EXPORT PARTITION ID '2020' TO TABLE iceberg_cas_export)
SETTINGS allow_experimental_insert_into_iceberg = 1

Expected behavior
The export succeeds.

Actual behavior

Code: 344. DB::Exception: Partition operation ALTER TABLE EXPORT PARTITION is not supported on a CAS disk yet (it clones parts file-by-file with no transaction, which would corrupt the clone); disk '__tmp_internal_…'. (SUPPORT_IS_DISABLED)

Key information

• Version: 26.6.2 antalya
• Regression currently asserts this rejection: cas/tests/export.py