ModelRouterMiddleware Throws Unclear KeyError When Classifier Returns Unconfigured Route

The wrap_model_call method (and its async counterpart) does not validate that the classifier response contains the expected 'model_route' answer or that the selected choice exists in the configured models dictionary before attempting to index. This leads to KeyError when the classifier output does not match configured routes.

1. Configure ModelRouterMiddleware with choices={'fast': ModelChoice(model=model, criteria='Simple tasks.')}. 2. Make classifier return ClassifierResponse with answers={'model_route': ChoiceAnswer(type='choice', choice='not-configured', ...)}. 3. Invoke agent with middleware. 4. Observe KeyError: 'not-configured' from self.models[answer.choice].

This change may raise ValueError for previously uncaught KeyError, which will surface as a different exception type. Ensure downstream code handles ValueError appropriately. Also, if the classifier legitimately returns a route not in the initial choices (e.g., dynamic routes), this fix would reject it; consider whether dynamic addition of routes is desired before merging. Rollback: revert to previous indexing code if unexpected breakage occurs.