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

引入 Canonical Model 和 ProtocolAdapter 架构，支持 OpenAI/Anthropic 协议间
无缝转换，统一 ProxyHandler 替代分散的 OpenAI/Anthropic Handler，简化
ProviderClient 为协议无关的 HTTP 发送器，Provider 新增 protocol 字段。

backend/internal/conversion/interface.go

@@ -0,0 +1,13 @@
+package conversion
+
+// InterfaceType 接口类型枚举
+type InterfaceType string
+
+const (
+	InterfaceTypeChat      InterfaceType = "CHAT"
+	InterfaceTypeModels    InterfaceType = "MODELS"
+	InterfaceTypeModelInfo InterfaceType = "MODEL_INFO"
+	InterfaceTypeEmbeddings InterfaceType = "EMBEDDINGS"
+	InterfaceTypeRerank    InterfaceType = "RERANK"
+	InterfaceTypePassthrough InterfaceType = "PASSTHROUGH"
+)