Closes #4067

Description

Fixes IsNull/NotNull binding silently dropping rows/files for a required field nested under an optional ancestor struct.
Related iceberg java/rust PRs:

• apache/iceberg#14270
• apache/iceberg-rust#3292

Bug:

• BoundIsNull/BoundNotNull.__new__ folded to AlwaysFalse()/AlwaysTrue() using only a field's own required flag.
• A required leaf can still be absent if an optional ancestor struct containing it is null
• plan_files() silently dropped matching files once IsNull/NotNull hit a required field under an optional parent.

Example:

# schema: s is optional, its only child x is required
# s = None for the one row -> s.x is absent too

files = table.scan(row_filter=IsNull("s.x")).plan_files()

len(files)  # 0  <-- empty, should contain the file with the s = None row

New behavior:

• IsNull/NotNull on a required field nested under an optional ancestor now correctly matches rows/files instead of silently pruning them.
• No public API changes.

Change

• pyiceberg/schema.py — add Schema.is_field_required_in_path(field_id), which walks the existing ancestor index to check the field and every ancestor is required. Mirrors Java's TypeUtil.ancestorFields/allAncestorFieldsAreRequired.
• pyiceberg/expressions/__init__.py — remove the leaf-only fold from BoundIsNull/BoundNotNull.__new__; do the fold in IsNull.bind()/NotNull.bind() instead, matching where Java does it (UnboundPredicate.bindUnaryOperation) — no change to BoundReference.
• tests/test_schema.py, tests/expressions/test_expressions.py — coverage for the fix.