feat(gemini): support cached token billing

setting/ratio_setting/cache_ratio.go

@@ -8,6 +8,8 @@ import (
 )
 
 var defaultCacheRatio = map[string]float64{
+	"gemini-3-flash-preview":              0.25,
+	"gemini-3-pro-preview":                0.25,
 	"gpt-4":                               0.5,
 	"o1":                                  0.5,
 	"o1-2024-12-17":                       0.5,