Refactor endpoint caching to be explicit and typed

[fderuiter]

Refactored the caching mechanism in the endpoint classes to address "Primitive Obsession" and "Implicit vs Explicit" architectural smells.

Key changes:

• Removed _cache_name property from EndpointABC and GenericEndpoint.
• Added _enable_cache boolean flag to EndpointABC (default False).
• Added _cache attribute to GenericEndpoint, strictly typed as Optional[Union[List[T], Dict[str, List[T]]]].
• Updated GenericEndpoint.__init__ to initialize _cache explicitly based on _enable_cache and requires_study_key.
• Refactored CacheMixin to use self._cache directly instead of dynamic attribute access.
• Updated FormsEndpoint, StudiesEndpoint, VariablesEndpoint, and IntervalsEndpoint to set _enable_cache = True instead of defining _cache_name.
• Updated EndpointProtocol to reflect these changes.

Verification:

• Created a temporary verification script to assert correct cache initialization.
• Ran existing unit tests for endpoints (test_studies_endpoint.py, test_forms_endpoint.py, etc.) which explicitly test caching behavior.
• Ran the full unit test suite to ensure no regressions.
• Verified strict typing and sync/async parity.

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PR created automatically by Jules for task 5961568543587308941 started by @fderuiter

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