Keep reco counts report tests on mock backends

[adamtheturtle]

Closes #3586.

Summary

• Keep the documented reco-count report endpoint in the mock.
• Run its dedicated contract tests against the mock backends only.
• Statically exclude its real-Vuforia combinations from the shared cross-cutting tests while retaining those tests on both mock backends.
• Remove the response-triggered skip logic, so unrelated real-service failures are not hidden.
• Document why the mock retains an endpoint which is permanently unavailable to this project's real-Vuforia test account.

Vuforia still documents the endpoint, and it previously worked against the real service. It now returns an HTML 404 for every database available to the test account, including a newly created database whose ID and server keys were read directly from Target Manager. The mock therefore keeps the useful documented behavior without pretending that this project can continue verifying it against the real service.

Validation

• uv run --group=dev prek run --all-files
• uv run --group=dev prek run --hook-stage pre-push --all-files
• dedicated reco-count report tests: 50 passed
• applicable cross-cutting reco-count cases: 48 passed, with 27 real-account combinations statically excluded