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/handler/provider_handler.go

@@ -26,10 +26,11 @@ func NewProviderHandler(providerService service.ProviderService) *ProviderHandle
 // CreateProvider 创建供应商
 func (h *ProviderHandler) CreateProvider(c *gin.Context) {
 	var req struct {
-		ID      string `json:"id" binding:"required"`
-		Name    string `json:"name" binding:"required"`
-		APIKey  string `json:"api_key" binding:"required"`
-		BaseURL string `json:"base_url" binding:"required"`
+		ID       string `json:"id" binding:"required"`
+		Name     string `json:"name" binding:"required"`
+		APIKey   string `json:"api_key" binding:"required"`
+		BaseURL  string `json:"base_url" binding:"required"`
+		Protocol string `json:"protocol"`
 	}
 
 	if err := c.ShouldBindJSON(&req); err != nil {
@@ -39,11 +40,17 @@ func (h *ProviderHandler) CreateProvider(c *gin.Context) {
 		return
 	}
 
+	protocol := req.Protocol
+	if protocol == "" {
+		protocol = "openai"
+	}
+
 	provider := &domain.Provider{
-		ID:      req.ID,
-		Name:    req.Name,
-		APIKey:  req.APIKey,
-		BaseURL: req.BaseURL,
+		ID:       req.ID,
+		Name:     req.Name,
+		APIKey:   req.APIKey,
+		BaseURL:  req.BaseURL,
+		Protocol: protocol,
 	}
 
 	err := h.providerService.Create(provider)