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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