Refactor endpoints for DRY and SOLID principles

[fderuiter]

This PR refactors the endpoint architecture to reduce code duplication and enforce SOLID principles.

Key changes:

1. GenericEndpoint Refactoring: Added a _get_client(is_async: bool) helper method to GenericEndpoint. This centralizes the logic for retrieving the appropriate client (sync or async) based on the execution context, adhering to the DRY principle and simplifying subclasses.
2. EdcEndpointMixin Cleanup: Removed the _build_path method from EdcEndpointMixin. The implementation was identical (in behavior) to GenericEndpoint._build_path, so removing it eliminates redundancy. EdcEndpointMixin now relies on the base implementation, which correctly handles the BASE_PATH defined in the mixin.
3. GenericListEndpoint & GenericListPathGetEndpoint Update: Updated these mixins to use the new _get_client helper, making the client retrieval logic consistent across the codebase.
4. RecordsEndpoint Consolidation: Refactored RecordsEndpoint to introduce a _create_impl method. This method encapsulates the common logic for creating records, which was previously duplicated in create and async_create. The public methods now delegate to this implementation, ensuring consistent behavior and reducing maintenance overhead.

These changes are structural only and do not alter the runtime behavior of the SDK. Tests have been run to verify no regressions were introduced.

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PR created automatically by Jules for task 18318603622383437127 started by @fderuiter

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