partition field names validation against schema field conflicts

[rutb327]

Closes #2272
Collaborator: @geruh

Rationale for this change

Implements the validation logic described in #2272 to match Java and Rust behavior for partition field name conflicts with schema fields.
This mirrors the method in Java checkAndAddPartitionName():
https://github.com/apache/iceberg/blob/4dbc7f578eee7ceb9def35ebfa1a4cc236fb598f/api/src/main/java/org/apache/iceberg/PartitionSpec.java#L392-L416

Identity transforms (sourceColumnID != null)- Allow schema field name conflicts only when sourced form the same field
Non-identity (sourceColumnID == null)- Disallow any schema field name conflicts.

In this PR isinstance(transform, (IdentityTransform, VoidTransform)) is used to achieve the same logic as Java’s sourceColumnID check.

Are these changes tested?

Yes, all existing tests pass and added a test covering validation scenarios.

Are there any user-facing changes?

Yes. Non-identity transforms can no longer use schema field names as partition field names.

[rutb327]

In Java all partition-schema validation goes through https://github.com/apache/iceberg/blob/4dbc7f578eee7ceb9def35ebfa1a4cc236fb598f/api/src/main/java/org/apache/iceberg/PartitionSpec.java#L392-L416 during table creation with partition specs, partition spec updates and also during schema evolution.
In Python the validation in https://github.com/apache/iceberg-python/blob/d1c6005ad05166ab0fb08d3c15ccdfd7568e8013/pyiceberg/table/update/spec.py only covered partition spec updates
So, I've added the validation to:

• https://github.com/apache/iceberg-python/blob/d1c6005ad05166ab0fb08d3c15ccdfd7568e8013/pyiceberg/partitioning.py
• https://github.com/apache/iceberg-python/blob/d1c6005ad05166ab0fb08d3c15ccdfd7568e8013/pyiceberg/table/update/schema.py

Are these the correct locations for the validation logic, or should they be placed elsewhere?

[dingo4dev]

Thanks for your work on this!

To improve readability and keep related code together, what are your thoughts on placing all the partition validation logic inside the partitioning.py file? Centralizing it there could make the validation process easier for future contributors to find and understand.

Let me know what you think! @kevinjqliu