fix: Validate SetStatisticsUpdate correctly (fixes #2865)

[ragnard]

Previously the pydantic @model_validator on SetStatisticsUpdate would fail because it assumed statistics was a model instance. In a "before"" validator that is not the case.

Use an "after" validator instead, where we can use instantiated and validated fields.

Before

>>> import pyiceberg.table.update
>>> pyiceberg.table.update.SetStatisticsUpdate.model_validate({'statistics': {'snapshot-id': 1234, 'file-size-in-bytes': 0, 'statistics-path': '', 'file-footer-size-in-bytes': 0, 'blob-metadata': []}})
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    pyiceberg.table.update.SetStatisticsUpdate.model_validate({'statistics': {'snapshot-id': 1234, 'file-size-in-bytes': 0, 'statistics-path': '', 'file-footer-size-in-bytes': 0, 'blob-metadata': []}})
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ragge/projects/github.com/ragnard/iceberg-python/.venv/lib/python3.14/site-packages/pydantic/main.py", line 716, in model_validate
    return cls.__pydantic_validator__.validate_python(
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        obj,
        ^^^^
    ...<5 lines>...
        by_name=by_name,
        ^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/ragge/projects/github.com/ragnard/iceberg-python/pyiceberg/table/update/__init__.py", line 191, in validate_snapshot_id
    data["snapshot_id"] = stats.snapshot_id
                          ^^^^^^^^^^^^^^^^^
AttributeError: 'dict' object has no attribute 'snapshot_id'

After

>>> import pyiceberg.table.update
>>> pyiceberg.table.update.SetStatisticsUpdate.model_validate({'statistics': {'snapshot-id': 1234, 'file-size-in-bytes': 0, 'statistics-path': '', 'file-footer-size-in-bytes': 0, 'blob-metadata': []}})
SetStatisticsUpdate(action='set-statistics', statistics=StatisticsFile(snapshot_id=1234, statistics_path='', file_size_in_bytes=0, file_footer_size_in_bytes=0, key_metadata=None, blob_metadata=[]), snapshot_id=1234)

Rationale for this change

Are these changes tested?

Yes, but only using the two-liners above.

Are there any user-facing changes?

No.

[kevinjqliu]

Thanks for the PR. I think we should just deprecate the top-level snapshot_id entirely.
For context, the "before model_validator` was added in 3b53edc#diff-769b43e1d8beaa86141f694679de2bbea3604a65f987a9acd7d9e9efca193b7eR181-R193 to maintain backwards compatibility and prep for deprecation

[kevinjqliu]

oops didnt mean to approve