🏛️ Architect: Centralize _raise_not_found Across Endpoints

[fderuiter]

🏛️ Design Change:
Centralized the _raise_not_found method inside EndpointABC and mapped its required signature into EndpointProtocol. Removed duplicated definitions from PathGetEndpointMixin and JobsEndpoint and utilized self._raise_not_found within FilterGetEndpointMixin.

♻️ DRY Gains:
Eliminated duplicated error-raising logic across 3 different files. The logic now lives in a single place (EndpointABC), guaranteeing uniform exception structures.

🛡️ Solidity:
Improved Explicit > Implicit design by having a centralized standard exception mechanism. Retained strict types and full sync/async parity since the logic resides in the shared base class.

⚠️ Breaking Changes:
None. Input parameters and behavior remains completely identical. Internally, exceptions utilize the MODEL metadata, meaning JobsEndpoint changed its exception string slightly to match the standard format, which is an intentional architectural alignment.

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PR created automatically by Jules for task 1361347405524891355 started by @fderuiter

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