test: add unit tests for projects billing fields list methods

- Add comprehensive unit tests for ListBillingFieldsV1Response model
- Test model validation with full, minimal, and empty data scenarios
- Test individual fields: accessors, deployments, tags, line_items
- Test serialization, deserialization, and immutability
- Test edge cases: large datasets, special characters, unicode
- Test deployment types and integration scenarios
- 26 passing tests with full coverage of billing fields endpoint

Author: lukeocodes