feat: bill OpenAI cache_write_tokens at cache-creation price with zero clamp

Parse OpenAI's native cache_write_tokens (chat prompt_tokens_details /
responses input_tokens_details), bill it at the cache-creation ratio, and
clamp the uncached prompt remainder at zero since cached + cache-write can
exceed prompt_tokens. Propagate the field through chat/responses/claude
format conversions and tiered expression billing (cc variable).

setting/ratio_setting/cache_ratio.go

@@ -81,6 +81,9 @@ var defaultCacheRatio = map[string]float64{
 }
 
 var defaultCreateCacheRatio = map[string]float64{
+	"gpt-5.6-sol":                         1.25,
+	"gpt-5.6-terra":                       1.25,
+	"gpt-5.6-luna":                        1.25,
 	"claude-3-sonnet-20240229":            1.25,
 	"claude-3-opus-20240229":              1.25,
 	"claude-3-haiku-20240307":             1.25,