feat: support cached input token billing

apps/api/internal/clients/gemini.go

@@ -519,7 +519,11 @@ func geminiUsage(raw map[string]any) Usage {
 	input := intFromAny(usageMap["prompt_tokens"])
 	output := intFromAny(usageMap["completion_tokens"])
 	total := intFromAny(usageMap["total_tokens"])
-	return Usage{InputTokens: input, OutputTokens: output, TotalTokens: total}
+	cachedInput := cachedInputTokensFromOpenAIUsage(usageMap)
+	if cachedInput > input && input > 0 {
+		cachedInput = input
+	}
+	return Usage{InputTokens: input, OutputTokens: output, CachedInputTokens: cachedInput, TotalTokens: total}
 }
 
 func geminiUsageMap(raw map[string]any) map[string]any {
@@ -527,8 +531,13 @@ func geminiUsageMap(raw map[string]any) map[string]any {
 	input := intFromAny(meta["promptTokenCount"])
 	output := intFromAny(meta["candidatesTokenCount"])
 	total := intFromAny(meta["totalTokenCount"])
+	cachedInput := intFromAny(firstPresent(meta["cachedContentTokenCount"], meta["cached_content_token_count"]))
 	if total == 0 {
 		total = input + output
 	}
-	return map[string]any{"prompt_tokens": input, "completion_tokens": output, "total_tokens": total}
+	usage := map[string]any{"prompt_tokens": input, "completion_tokens": output, "total_tokens": total}
+	if cachedInput > 0 {
+		usage["prompt_tokens_details"] = map[string]any{"cached_tokens": cachedInput}
+	}
+	return usage
 }