Supply mock credentials from the backend adapter

[adamtheturtle]

The mock VWS/VuMark credential fixtures currently read the same settings as real Vuforia. Mock runs therefore require a secrets file and can use real account credentials when one is present.

Move settings classes and readers into the adapter's real_credentials module. The adapter supplies generated credentials for both mock backends and implementation tests, and invokes the existing readers only for the real backend. It reuses the active internal selector without adding another backend matrix. The shared credentials module now contains only credential data types.

Part of #3738.

Validation:

• Full non-Docker suite without vuforia_secrets.env: 1,980 passed, 839 skipped, with real-service execution disabled.
• Five permanent credential-policy cases: both mock selections work with invalid settings, standalone mock tests need no settings, and the real VWS/VuMark and Model Target readers load an isolated file containing only the tracked example credentials. No real-service requests are made by these cases.
• Independent consumer: 8 passed at custom URLs with Flask, native-adapter and settings-loader imports blocked; all 23 shared setup modules import successfully.
• All 2,819 existing test cases and IDs preserved; five policy cases added. AST comparison preserves credential data types, settings classes, and real-reader bodies, apart from renaming former fixture functions and removing their pytest decorators.
• 100% coverage for shared credential types, real readers, root configuration, and the new policy tests; every added executable adapter line covered.
• Pre-commit/pre-push checks, type checking, CI partition and manifest validation, changed-file Pylint, and git diff --check.