[codex] Improve Model Target auth fidelity

[adamtheturtle]

Summary

• Match Model Target OAuth and bearer-token failure responses more closely to real Vuforia.
• Return JWT-shaped mock access tokens and require JWT-shaped bearer tokens on Model Target routes.
• Document the remaining token validation limitation and add a news fragment for Improve Model Target Web API authentication fidelity #3192.

Validation

• uv run --extra dev pytest tests/mock_vws/test_model_target_web_api.py::TestAuthentication -q
• uv run --extra dev pytest tests/mock_vws/test_model_target_web_api.py --skip-real -q
• uv run --extra dev pytest tests/mock_vws/test_requests_mock_usage.py::TestModelTargetWebAPI tests/mock_vws/test_respx_mock_usage.py::TestModelTargetWebAPI tests/mock_vws/test_flask_app_usage.py::TestModelTargetWebAPI -q
• Pre-commit and pre-push hooks passed during commit/push.