Support State-Based Model Target fields - #3437

What changed

• accept stateBasedConfigurationJsonString on standard and advanced Model Target models
• accept optional per-view states, treating omission as all configured states
• validate configuration JSON shape, view-state field types, and ensure selected states are declared by the configuration
• run the new creation and validation coverage against both the in-memory and Flask-backed mock backends
• document the remaining mock-only limitation and add a changelog fragment

Why

This is a focused step toward #3348. The mock previously stored these fields without understanding them, so invalid state selections could not be exercised in tests.

Real-service verification is not included because the available test account lacks the State-Based Model Target OAuth scopes; that limitation is documented explicitly.

Validation

• uv run --extra=dev pytest -q tests/mock_vws/test_model_target_web_api.py --skip-real — 421 passed, 195 skipped
• uv run --extra=dev pyright src/mock_vws/_model_target_web_api.py tests/mock_vws/fixtures/vuforia_backends.py tests/mock_vws/test_model_target_web_api.py
• uv run --extra=dev mypy src/mock_vws/_model_target_web_api.py tests/mock_vws/fixtures/vuforia_backends.py tests/mock_vws/test_model_target_web_api.py
• repository pre-commit hooks for all changed files
• pre-push hooks, including partition checks and full type-checker suite