Mock exhausted Model Target training allowance - #3463

Closes #3462

What changed

• add an opt-in model_target_training_allowance_exceeded setting to MockVWS
• return Vuforia-style HTTP 422 TRAINING_ALLOWANCE_EXCEEDED responses for standard and advanced dataset creation
• support the same behavior in the Flask/Docker backend through MODEL_TARGET_TRAINING_ALLOWANCE_EXCEEDED=true
• document both configuration paths and add a release note
• assign the new test module to the CI partition matrix

The setting affects dataset creation only. Existing status, download, and deletion behavior is unchanged. Live Vuforia tests are not skipped when its allowance is exhausted.

Why

An exhausted Vuforia training allowance could previously be exercised only against a live account after its quota had been consumed. Client handling for that state can now be tested deterministically without weakening the real-Vuforia contract tests.

Validation

• uv run pytest -q --skip-real tests/mock_vws/test_model_target_web_api.py tests/mock_vws/test_model_target_training_allowance.py — 518 passed, 244 skipped
• focused generation failure/warning and exhausted-allowance tests — 13 passed
• uv run pre-commit run --all-files
• pre-push partition, mypy, pyright, verifytypes, ty, pyrefly, and manifest checks