Decouple shared API tests from concrete mock implementations #3738

Part of #3737.

The shared API suite currently couples backend selection, setup, and helper operations in tests/mock_vws/fixtures/vuforia_backends.py and related modules. Decouple these while both implementations remain in this repository, so the suite can later run with a backend supplied by another repository.

Acceptance criteria:

• Define a small typed backend/fixture contract for API clients and endpoint URLs, database setup/reset, and mock controls such as adding databases and seeding recognition counts.
• Separate backend selection from setup/lifecycle fixtures. Keep backend identities and selection results inside adapters; shared setup fixtures must return None (or yield None with teardown), so shared autouse dependencies can use None instead of object. Preserve backend parametrization, skip policy, and setup/cleanup behavior. Do not merely annotate the current enum-returning fixtures as None.
• Keep implementation setup and concrete imports in backend adapters. Shared assertions and helpers must not import Flask or branch on concrete backend identities.
• Replace hardcoded Vuforia endpoint URLs in client/prepared-request helpers with supplied URLs, including Model Target requests and report downloads.
• Use synthetic credentials for mock runs; keep real-Vuforia credentials and account-specific restrictions in the explicit real backend.
• Preserve shared coverage for in-memory and Flask adapters, mock-only cases, and applicable real-Vuforia verification. Keep tests of implementation-specific behavior local to their implementation.
• Verify that the shared suite runs against both existing mock adapters, with existing coverage requirements and CI partition checks preserved.

Implemented in merged PRs #3759 and #3760: shared API fixtures load independently of adapter registration; backend selection stays internal to adapters; shared setup/lifecycle fixtures yield None, replacing the transitional object annotations. The remaining decoupling criteria above still apply.

Completion point: the same shared assertions run through either adapter without importing either implementation in the shared suite. Packaging and the repository move are separate follow-up work.