Separate backend selection from shared setup fixtures

[adamtheturtle]

The shared backend setup fixtures currently expose the adapter's VuforiaBackend selection as their result, requiring shared autouse fixtures to accept an ignored object value.

Keep backend selection and skip policy in adapter-internal fixtures. Add separate public setup fixtures which set up and tear down the selected backend and yield None. Adapter operation providers depend on that setup and read the internal selector, while all 23 shared directory setup fixtures now accept None.

Part of #3738. Follows #3759.

Validation:

• Both mock adapters: 1,494 passed, 750 skipped in the affected shared directories; dataset coverage adds 36 passed, 11 skipped. Real-service execution was disabled.
• All 2,819 collected test cases and IDs preserved. AST comparison confirms existing adapter helpers and operation bodies are unchanged apart from selector references and discarding the setup result.
• Direct lifecycle probes verify None results, one setup and teardown per test, and backend skips before setup. Offline real-policy probes verify rate-limit/capability defaults and signed Model Target skips before credential lookup, including the explicit signing opt-in. These use stubbed setup and credentials and make no live requests.
• Independent consumer: 8 passed at custom URLs with a None-yielding setup fixture and Flask/native-adapter imports blocked, using the repository's runtime type-checking configuration.
• 100% coverage for all 23 shared setup modules; every added executable adapter line covered.
• Pre-commit/pre-push checks, including type checking, manifest and CI partition validation; changed-file Pylint; git diff --check.