fix: track batch position delete references

[u70b3]

Summary

• collect referenced data-file paths when writing position deletes as an Arrow batch
• update referenced-path metadata only after the underlying write succeeds
• reject null batch inputs and null file paths
• cover single-file, multi-file, mixed, empty, and invalid batches

Testing

• pre-commit run --files src/iceberg/data/position_delete_writer.cc src/iceberg/test/data_writer_test.cc
• build-gcc14/src/iceberg/test/data_test (157 tests passed)
• git diff --check

This contribution was developed with assistance from OpenAI Codex. The implementation and tests were reviewed and verified by the author.