test(auth): guard flagship cases cover every ranked provider

post_auth_model_selection_picks_flagship_for_every_ranked_provider used
a hand-written cases table with no check that it covered all of
RANKED_PROVIDER_IDS. A newly ranked provider could be added without a
case and the 'for_every_ranked_provider' claim would silently stop
holding. Assert the covered set equals RANKED_PROVIDER_IDS so the table
cannot drift out of sync with the classification guard.