refactor: cleanup endpoints and mixins (DRY/SOLID)

[fderuiter]

This PR implements several architectural cleanups identified during the audit:

1. Redundancy Removal: The JobsEndpoint had get and async_get methods that merely called super() with the same arguments. These were removed to rely on the parent class implementation, adhering to DRY.
2. Dead Code Removal: The CreateEndpointMixin (in imednet/core/endpoint/mixins/create.py) was not used by any endpoint (records use RecordCreateOperation) or tests. It has been deleted.
3. Utility Extraction: The client-side filtering logic in SubjectsEndpoint (comparing IDs as strings) was extracted into a reusable, generic function filter_by_attribute in imednet/utils/filters.py. This improves cohesion and testability.

These changes strictly follow SOLID principles and the "Architect" persona's directives for a modular and clean codebase.

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PR created automatically by Jules for task 8663856002586161599 started by @fderuiter

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