feat(ai): allow provider-specific numeric fields on UsageTotals.costDetails

The JSDoc promised "loosely typed" costDetails to accommodate provider
divergence (BYOK upstream costs, cache discounts, per-tier rates) but
the declared shape only allowed two OpenRouter-specific keys, so any
other adapter would have been forced to `as any`-cast to report its
own breakdown. Add a numeric index signature so additional keys are
type-legal without a cast, matching the documented intent.

Picked up in CodeRabbit review of TanStack#469.

Author: season179

packages/typescript/ai/src/types.ts

@@ -829,11 +829,14 @@ export interface UsageTotals {
    * Provider-reported cost breakdown. Loosely typed because providers
    * disagree on what to expose (BYOK upstream costs, cache discounts,
    * per-tier rates, ...) and locking the shape down would force every
-   * new adapter to either lie or back-fill.
+   * new adapter to either lie or back-fill. The named fields are
+   * whatever OpenRouter currently reports; other adapters may report
+   * numeric fields under their own keys via the index signature.
    */
   costDetails?: {
     upstreamInferenceCost?: number | null
     cacheDiscount?: number | null
+    [key: string]: number | null | undefined
   }
 }