resolveModelPricing() in analytics route fell back to
Object.keys(providerPricing)[0] when a model had no pricing
entry. For OpenRouter, the defaults layer always contributes
an 'auto' record as the first key, so every :free model was
charged at that arbitrary rate in the analytics dashboard.
Fix: short-circuit :free models to return null before the
last-resort fallback, and remove the Object.keys(...)[0]
arbitrary-substitution fallback.
Closes#9054
4aa46e659a