Fix cache token accounting and tiered pricing

frontend/src/features/models/utils/tiered-pricing-multipliers.ts

@@ -0,0 +1,17 @@
+export function roundTierPrice(value: number): number {
+  return Number.parseFloat(Math.max(0, value).toFixed(6))
+}
+
+export function cachePriceFromInputMultiplier(inputPrice: number, multiplier: number): number {
+  const normalizedMultiplier = Number.isFinite(multiplier) && multiplier >= 0 ? multiplier : 0
+  return roundTierPrice(inputPrice * normalizedMultiplier)
+}
+
+export function cacheMultiplierFromPrice(
+  inputPrice: number,
+  cachePrice: number | undefined,
+  fallback: number,
+): number {
+  if (cachePrice == null || inputPrice <= 0) return fallback
+  return roundTierPrice(cachePrice / inputPrice)
+}