Type decoded JWT payloads as JSON

[adamtheturtle]

Summary

• validate decoded JWT payloads with the existing recursive JSON-object guard
• return a string-keyed JSON mapping instead of an unchecked Mapping[object, object]
• remove the redundant generic mapping guard and the object annotation on the scope value

Validation

• uv run --group=dev prek run --all-files --hook-stage pre-commit --no-fail-fast
• uv run --group=dev prek run --all-files --hook-stage pre-push --no-fail-fast
• uv run --group=dev pytest -q tests/mock_vws/test_model_target_web_api.py::TestMockOnlyOAuth2EdgeCases (9 passed)