fix: correct NOT STARTS WITH projection for truncated partitions

[GayathriSrividya]

closes #3493

Summary

Fixes incorrect projection of NOT STARTS WITH predicates for truncated string/binary partition fields. The current implementation unsafely truncates the filter literal without checking its length relative to the truncate width.

Root Cause

The TruncateTransform.project method calls _truncate_array which blindly truncates the literal for both STARTS WITH and NOT STARTS WITH predicates:

elif isinstance(pred, BoundNotStartsWith):
    return NotStartsWith(Reference(name), _transform_literal(transform, boundary))

For NOT STARTS WITH "hello" with truncate[2], this produces:

• Current (unsafe): NOT STARTS WITH "he"
• Problem: The truncated partition contains all values starting with "he" (from "hello", "heat", "hear", etc.), so we cannot safely exclude all non-"hello" rows

Solution

Add special handling for BoundNotStartsWith in the project method following the Java/Go reference behavior:

• prefix_length < truncate_width: Keep original NOT STARTS WITH literal (safe)
• prefix_length == truncate_width: Project to != instead (safe equality check)
• prefix_length > truncate_width: Return None (no inclusive projection possible)

pyiceberg/transforms.py

• Add explicit NOT STARTS WITH handling before calling _truncate_array
• Check literal length vs truncate width and apply correct projection rules

tests/test_transforms.py

• Update test_projection_truncate_string_not_starts_with to expect None (prefix_length > width is unsafe)
• Add test_projection_truncate_string_not_starts_with_shorter_literal (prefix_length == width → !=)
• Add test_projection_truncate_string_not_starts_with_original_literal (prefix_length < width → original)

Validation

• make lint ✓ (all pre-commit hooks pass)
• pytest tests/test_transforms.py → 280 passed ✓
• All 13 string truncate projection tests pass