[adamtheturtle]

Towards #3738. Depends on #3820.

Move real Vuforia cleanup, request-timeout and training-allowance handling, setup contexts, and the live client's polling default into real_backend.py. The selection plugin delegates to this module while retaining fixture registration, backend identities, credential selection and collection restrictions.

Preserve the existing retry limits, safe-request policy, exception handling and live-service behavior. Remove the two default-argument suppressions that the extracted public override no longer needs.

Validation:

• AST audit confirms every setup, fixture, policy and client definition is unchanged apart from renamed delegated references.
• Existing retry, response-policy, timeout and credential tests: 43 passed.
• Isolated installed-wheel consumer passes.
• All pre-commit and pre-push hooks, partition checks and type checkers pass; changed-file Pylint reports 10/10.

This leaves Flask and real-service setup in separate modules without changing the shared contract assertions.