Refactor: Implement Strategy pattern for params and study key handling

[fderuiter]

Refactored ParamMixin and endpoint parameter processing to use the Strategy pattern.

• Introduced StudyKeyStrategy protocol with implementations: PopStudyKeyStrategy, KeepStudyKeyStrategy, OptionalStudyKeyStrategy.
• Introduced MappingParamProcessor for declarative parameter mapping.
• Updated UsersEndpoint and RecordsEndpoint to use new strategies, removing ad-hoc processor classes and boolean flags.
• Extracted validate_record_entry logic to imednet/validation/cache.py to improve cohesion.
• Updated StudiesEndpoint to use OptionalStudyKeyStrategy.
• Updated unit tests to align with new architecture.

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PR created automatically by Jules for task 10998208052101135393 started by @fderuiter

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