Fix TruncateTransform.satisfies_order_of for different widths (#3680)

[hedger9487]

Closes #3680

Rationale for this change

TruncateTransform.satisfies_order_of previously accessed self.source_type (and other.source_type), which raised AttributeError: 'TruncateTransform' object has no attribute '_source_type' whenever comparing truncate transforms with different widths.

Per the Iceberg spec and matching the Java reference implementation, ordering satisfaction between two truncate transforms depends purely on the width comparison (self.width >= other.width).

This PR:

1. Updates TruncateTransform.satisfies_order_of to check isinstance(other, TruncateTransform) and compare self.width >= other.width.
2. Adds unit tests covering same/different width comparisons and cross-transform comparisons.

Are these changes tested?

Yes, added test_truncate_satisfies_order_of in tests/test_transforms.py. All tests and pre-commit linters pass cleanly.

Are there any user-facing changes?

No.

[Copilot]

Pull request overview

Fixes a runtime AttributeError in TruncateTransform.satisfies_order_of when comparing truncate transforms with different widths by removing reliance on the unset source_type, aligning behavior with the Iceberg reference implementation.

Changes:

• Simplified TruncateTransform.satisfies_order_of to check isinstance(other, TruncateTransform) and compare widths (self.width >= other.width).
• Added unit tests covering width comparisons and ensuring non-truncate transforms return False.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pyiceberg/transforms.py	Fixes satisfies_order_of to avoid accessing an uninitialized private attribute and bases ordering satisfaction on width comparison.
tests/test_transforms.py	Adds regression tests for truncate-width comparisons and cross-transform comparisons.

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[rambleraptor]

It doesn't appear that we use source_type anywhere after this change. Can you take this opportunity to remove it?

[hedger9487]

Makes sense! Took this opportunity to clean up _source_type and the source_type property as well, and expanded the unit test suite to verify cross-transform comparisons and attribute cleanup.

Thanks for the review, @rambleraptor!