Remove unused `DatasetStatus.PENDING` and `DatasetStatus.STALE`

[ilongin]

Two values in DatasetStatus are defined but never written by any code in the repo:

• PENDING = 2 (src/datachain/dataset.py:263)
• STALE = 6 (src/datachain/dataset.py:266)

Grep shows only reads:

PENDING - appeared in a defensive guard list at catalog.complete_dataset_version ([CREATED, PENDING]) and in the wipe-routing list in catalog.remove_dataset_version. Both removed in the soft-delete PR.

STALE - read in:

• DatasetVersion.is_final_status() (dataset.py)
• get_dataset_versions_to_clean() cleanup query (metastore.py)
• catalog.remove_dataset_version routing (removed in the soft-delete PR)

No production code sets either status. Dead branches in the state machine.

Tasks

• Confirm no setter exists for either status
• Remove PENDING and STALE from the DatasetStatus enum
• Remove dead read paths that reference them
• Update docstrings / status table to drop them

[shcheklein]

Let's also check Studio code, just in case.