Introduces LookupPricingAt() to resolve model pricing at a specific timestamp instead of always using current prices. This is important for accurate cost calculations when analyzing historical sessions where pricing may have changed. Changes: - Add modelPricingVersion struct with EffectiveFrom timestamp - Add defaultPricingHistory map for versioned pricing entries - Update LookupPricing to delegate to LookupPricingAt(model, time.Now()) - Add comprehensive tests for time-windowed pricing lookup The infrastructure supports future pricing changes by adding entries to defaultPricingHistory with their effective dates. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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