🧹 Extract duplicated NotImplementedError handling in DeferredMultiNodeBackend

[edithatogo]

🎯 What: Extracted duplicate exception-raising logic into a single _raise_deferred helper in DeferredMultiNodeBackend.
💡 Why: Reduces duplicate code and improves maintainability of the fallback implementation.
✅ Verification: Pre-commit hooks passed and full test suite run via pytest with no regressions.
✨ Result: A cleaner implementation with identical behavior.

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PR created automatically by Jules for task 18079879394769044546 started by @edithatogo

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