🛠️ Refactor: Endpoints - Explicit Study Key Strategy

[fderuiter]

This PR refactors how studyKey handling strategies are defined and resolved for SDK endpoints.

Problem: Endpoints were relying on implicit class-level flags (_pop_study_filter: bool and _missing_study_exception: type[Exception]) which ParamMixin used dynamically to instantiate a StudyKeyStrategy via backward compatibility logic. This violated the "Explicit > Implicit" rule and obscured the Strategy pattern.

Solution:

1. Removed _pop_study_filter and _missing_study_exception from EndpointProtocol and ParamMixin.
2. Updated ParamMixin.study_key_strategy to resolve self.STUDY_KEY_STRATEGY directly, defaulting to KeepStudyKeyStrategy or OptionalStudyKeyStrategy based solely on requires_study_key.
3. Updated all relevant endpoints (Codings, Forms, Intervals, Records, Sites, Users, Variables) to explicitly instantiate and assign the correct PopStudyKeyStrategy to their STUDY_KEY_STRATEGY attribute.

Verification:

• Tests pass: poetry run pytest
• Type checks pass: poetry run mypy src/imednet
• Formatting checks pass: poetry run black . and poetry run ruff check --fix .

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PR created automatically by Jules for task 10957207828714997985 started by @fderuiter

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