feat(antigravity): 支持 Gemini cachedContentTokenCount 映射到 Claude cache_read_input_tokens

- 在 GeminiUsageMetadata 添加 CachedContentTokenCount 字段
- 修正 token 映射：Gemini promptTokenCount 包含缓存，Claude input_tokens 不包含
- 流式和非流式响应均已支持

backend/internal/pkg/antigravity/response_transformer.go

@@ -232,10 +232,14 @@ func (p *NonStreamingProcessor) buildResponse(geminiResp *GeminiResponse, respon
 		stopReason = "max_tokens"
 	}
 
+	// 注意：Gemini 的 promptTokenCount 包含 cachedContentTokenCount，
+	// 但 Claude 的 input_tokens 不包含 cache_read_input_tokens，需要减去
 	usage := ClaudeUsage{}
 	if geminiResp.UsageMetadata != nil {
-		usage.InputTokens = geminiResp.UsageMetadata.PromptTokenCount
+		cached := geminiResp.UsageMetadata.CachedContentTokenCount
+		usage.InputTokens = geminiResp.UsageMetadata.PromptTokenCount - cached
 		usage.OutputTokens = geminiResp.UsageMetadata.CandidatesTokenCount
+		usage.CacheReadInputTokens = cached
 	}
 
 	// 生成响应 ID