🛠️ Refactor: SDK Core - Centralize Endpoint Path Building and Not Found Logic (DRY)

[fderuiter]

🛠️ Refactor: SDK Core - Centralize Endpoint Path Building and Not Found Logic (DRY)\n\n♻️ DRY Gains:\nCollapsed identical path building and 404 raising logic across PathGetEndpointMixin, ListEndpointMixin, FilterGetEndpointMixin, JobsEndpoint, and RecordsEndpoint into EndpointABC. \n\n🧱 SOLID:\nEnforced Interface Segregation and Dependency Inversion by requiring subclasses to implement base path properties while abstracting validation away from the mixins. Mixins now only depend on _get_endpoint_path and _raise_not_found rather than handling raw path generation.\n\n📉 Diff:\nRemoved duplicated generic path strings, redundant checks, and inline imports. No changes to observable behavior.

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PR created automatically by Jules for task 12189503407201544417 started by @fderuiter

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