fix: improve cost tracking accuracy for modern models

- Added support for reasoning tokens in cost calculations.
- Fixed DeepSeek cache-write token mapping (PromptCacheMissTokens).
- Improved CalculateCost debug logging to trace all pricing variables.

internal/utils/registry.go

@@ -34,9 +34,10 @@ func FetchRegistry() (*models.ModelRegistry, error) {
 	return &models.ModelRegistry{Providers: providers}, nil
 }
 
-func CalculateCost(registry *models.ModelRegistry, modelID string, promptTokens, completionTokens, cacheRead, cacheWrite uint32) float64 {
+func CalculateCost(registry *models.ModelRegistry, modelID string, promptTokens, completionTokens, reasoningTokens, cacheRead, cacheWrite uint32) float64 {
 	meta := registry.FindModel(modelID)
 	if meta == nil || meta.Cost == nil {
+		log.Printf("[DEBUG] CalculateCost: model %s not found or has no cost metadata", modelID)
 		return 0.0
 	}
 
@@ -61,5 +62,8 @@ func CalculateCost(registry *models.ModelRegistry, modelID string, promptTokens,
 		cost += float64(cacheWrite) * (*meta.Cost.CacheWrite) / 1000000.0
 	}
 
+	log.Printf("[DEBUG] CalculateCost: model=%s, uncached=%d, completion=%d, reasoning=%d, cache_read=%d, cache_write=%d, cost=%f (input_rate=%f, output_rate=%f)", 
+		modelID, uncachedTokens, completionTokens, reasoningTokens, cacheRead, cacheWrite, cost, meta.Cost.Input, meta.Cost.Output)
+
 	return cost
 }