fix: add Field alias for SetPartitionStatisticsUpdate.partition_stati…

[shiwk]

Rationale for this change

Add Field(alias="partition-statistics") to SetPartitionStatisticsUpdate.partition_statistics to ensure correct serialization/deserialization with the hyphenated key format.

Problem

The partition_statistics field was missing an explicit Field alias, which means:

• When serializing with model_dump(by_alias=True), it would use the Python attribute name partition_statistics (with underscore) instead of the Iceberg specification format partition-statistics (with hyphen)
• This causes incompatibility with the Iceberg table metadata format specification

Solution

Added Field(alias="partition-statistics") to ensure:

• Proper serialization to JSON/dict using hyphenated key names that comply with Iceberg spec
• Correct deserialization when parsing external metadata with hyphenated keys
• Consistency with other similar fields in the codebase (e.g., snapshot_ids with alias snapshot-ids)

Are these changes tested?

Yes. Added verification in test_set_partition_statistics_update() to validate that:

1. The update object serializes to JSON with the correct "partition-statistics" key
2. The key is present in the serialized output

Are there any user-facing changes?

No. This is an internal fix to ensure metadata serialization format compliance. The change is transparent to users and improves interoperability with the Iceberg specification.