Two changes that together prevent a caller from smuggling unauthorized
models past the API key's allowlist via per-request router overrides.
1. ``_enforce_key_and_fallback_model_access``: also walk fallback models
nested inside ``router_settings_override.fallbacks`` /
``context_window_fallbacks`` / ``content_policy_fallbacks``.
``route_llm_request.py`` promotes those to per-request kwargs after
auth, so without this they bypassed the model allowlist entirely.
New ``iter_router_fallback_model_names`` helper extracts leaf names
from both the simple top-level shape (str | {"model": str}) and the
nested router-config shape ({primary: [fallbacks]}). The two fallback
validation loops are unified — every name (top-level + override) is
deduplicated and validated once via ``can_key_call_model`` +
``is_valid_fallback_model``.
2. ``route_request``: strip router-internal ``mock_testing_*`` flags
from user-supplied data. These are testing-only flags that
deterministically force the router into fallback logic by raising a
synthetic ``InternalServerError`` etc. Combined with override
fallbacks they made the smuggling path trivially exploitable. Test
code that calls the router directly bypasses the strip and is
unaffected. The strip list is derived from ``MockRouterTestingParams``
so a new ``mock_testing_*`` flag added to that dataclass is
automatically covered.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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