fix: allow beta citation file_id in round-tripped params

[nightcityblade]

Fixes #1450

Summary

• add optional file_id to BetaCitationPageLocationParam so beta citation page locations can be round-tripped from response -> request
• add a transform test covering a known optional field with None

Testing

• python -m ruff check src/anthropic/types/beta/beta_citation_page_location_param.py tests/test_transform.py
• pytest -q tests/test_transform.py -k known_optional_none_field_is_preserved
• pytest -q tests/test_transform.py