refactor list_subjects_json in prep for fastapi

[RayBB]

No description provided.

[RayBB]

AI review

PR #11987 Review Summary
What it does: Refactors list_subjects_json in openlibrary/plugins/openlibrary/lists.py in preparation for fastapi migration.
Changes:

• Renamed get_subjects() → get_subjects_data() and made it a @staticmethod
• Made _process_subject() a @staticmethod
• Fixed a bug where loop variable key shadowed the key parameter, causing incorrect links
• Changed self._process_subject(s) → list_subjects_json._process_subject(s)
  Risks: None identified.
  The refactor is safe and the class reference is valid.