disallow non-identity partition fields with schema field names

[rutb327]

Closes 2272

Rationale for this change

Implements the validation logic described in 2272to 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.

[kevinjqliu]

hey @rutb327 could you open this PR against the apache/iceberg-python repo?

[kevinjqliu]

thanks for this test! could you add another one to check that the same check is applied with creating the table with both schema and partition spec?

something like

    schema = Schema(
        NestedField(1, "id", LongType(), required=False),
        NestedField(2, "event_ts", TimestampType(), required=False),
        NestedField(3, "another_ts", TimestampType(), required=False),
        NestedField(4, "str", StringType(), required=False),
    )
   partition_spec = PartitionSpec(
        PartitionField(source_id=1, field_id=1000, transform=IdentityTransform(), name="id"), spec_id=1
    )
    table = _create_table_with_schema(catalog, schema, "2", partition_spec)

[kevinjqliu]

and perhaps add a test for when the partition field is already there and we try to add a new schema field which will conflict

[rutb327]

I have raised the PR against the apache/iceberg-python. I will add these tests