refactor: 实现 ConversionEngine 协议转换引擎，替代旧 protocol 包

- 新增 ConversionEngine 核心引擎，支持 OpenAI 和 Anthropic 协议转换
- 添加 stream decoder/encoder 实现
- 更新 provider client 支持新引擎
- 补充单元测试和集成测试
- 更新 specs 文档

backend/internal/conversion/anthropic/stream_decoder.go

@@ -251,7 +251,7 @@ func (d *StreamDecoder) processMessageDelta(data []byte) []canonical.CanonicalSt
 	}
 
 	if d.accumulatedUsage != nil {
-		d.accumulatedUsage.OutputTokens += raw.Usage.OutputTokens
+		d.accumulatedUsage.OutputTokens = raw.Usage.OutputTokens
 	}
 
 	return []canonical.CanonicalStreamEvent{