fix: respect nullability in nested struct accessors

[advancedxy]

Summary

• Carry schema nullability alongside each nested accessor position, including the leaf field.
• Return null when an optional parent struct is null, and report an error when a required parent or leaf field is null.
• Preserve the direct access paths for shallow fields while handling deeper paths, non-struct parents, and field errors.
• Extend accessor tests across one- to four-level paths and align the evaluator test schema with its null input.

Testing

• pre-commit on all changed files: passed
• schema_test, expression_test, arrow_test, and eval_expr_test: 4/4 passed

AI assistance

OpenAI Codex assisted with the implementation, refactoring, and test scaffolding. The changes were reviewed against the schema accessor behavior and validated with the tests above.