Type Model Target request handling as recursive JSON

[adamtheturtle]

Issue

Model Target request validation used Any throughout the decoded JSON tree. That hid the actual container shapes from Pyrefly and required 19 ignores around ordinary dictionary, list, string, and number operations. The view helper also retained a branch for non-object values even though the request boundary normalizes every such value before calling it; its only test had to bypass two lint rules to call the private helper directly.

Change

Use the existing recursive JSONValue domain type at the response and request boundaries, then narrow nested arrays and objects before inspecting them. OAuth and validation response bodies are also constructed as recursive JSON values. Remove the unreachable view-helper branch and its implementation-detail test. This removes 21 suppression lines without changing HTTP behavior.

Validation

• uv run --group=dev prek run --all-files --hook-stage pre-commit
• uv run --group=dev prek run --all-files --hook-stage pre-push
• uv run --group=dev coverage run -m pytest --skip-backend=real -n auto . - 1,947 passed, 843 skipped
• changed production module: 100% local coverage

The local aggregate coverage command reports 99% because real-Vuforia-only and platform paths are skipped locally; the repository hosted coverage job combines those partitions.