Package the shared API contract suite for reuse from another repository #3739

Part of #3737.

Blocked by #3738.

Make the shared API contract suite an installable Python package maintained in this repository, so the server repository can consume one authoritative copy of the tests.

Proposed layout: a separately buildable contract-tests/ subproject with a unique import namespace such as vws_mock_contract_tests. This package is a development dependency of consumers, not a server runtime dependency. It can initially be installed from a Git dependency pinned to a commit and subdirectory; a separate PyPI release is optional.

Acceptance criteria:

• Package shared API test modules, fixtures, helpers, and required assets. Keep core interception tests and Flask/deployment-specific tests outside the shared package.
• Provide an explicitly loadable pytest plugin for shared fixtures. Backend adapters supplied by each consuming repository must also load explicitly; do not rely on a consumer's local conftest.py applying to installed tests.
• Document the setup/lifecycle contract established in Decouple shared API tests from concrete mock implementations #3738: shared setup fixtures return or yield None, while backend selection remains internal to the adapter. Verify this contract in the separate consumer smoke run.
• Support collecting the installed suite with pytest --pyargs and document installation, invocation, adapter requirements, and version/revision compatibility.
• Verify the built package from a separate working directory, without the core repository's tests or root conftest.py on the import path. Include a consumer smoke run through the existing Flask adapter.
• Run the same packaged suite in core CI, with collection/partition checks and coverage attribution updated appropriately, so there is no second maintained copy of the assertions.
• Ensure mock suite installation and execution require no real-Vuforia secrets and do not import Flask unless the chosen adapter requires it.

Completion point: another project can install the suite and run it with an explicit backend plugin. The server extraction is separate work.