fix: prevent sessionHash collision for different users with same messages

Mix SessionContext (ClientIP, UserAgent, APIKeyID) into
GenerateSessionHash 3rd-level fallback to differentiate requests
from different users sending identical content.

Also switch hashContent from SHA256-truncated to XXHash64 for
better performance, and optimize Trie Lua script to match from
longest prefix first.

backend/internal/handler/gateway_handler.go

@@ -203,6 +203,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 	}
 
 	// 计算粘性会话hash
+	parsedReq.SessionContext = &service.SessionContext{
+		ClientIP:  ip.GetClientIP(c),
+		UserAgent: c.GetHeader("User-Agent"),
+		APIKeyID:  apiKey.ID,
+	}
 	sessionHash := h.gatewayService.GenerateSessionHash(parsedReq)
 
 	// 获取平台：优先使用强制平台（/antigravity 路由，中间件已设置 request.Context），否则使用分组平台
@@ -962,6 +967,11 @@ func (h *GatewayHandler) CountTokens(c *gin.Context) {
 	}
 
 	// 计算粘性会话 hash
+	parsedReq.SessionContext = &service.SessionContext{
+		ClientIP:  ip.GetClientIP(c),
+		UserAgent: c.GetHeader("User-Agent"),
+		APIKeyID:  apiKey.ID,
+	}
 	sessionHash := h.gatewayService.GenerateSessionHash(parsedReq)
 
 	// 选择支持该模型的账号

backend/internal/handler/gemini_v1beta_handler.go

@@ -233,6 +233,13 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	if sessionHash == "" {
 		// Fallback: 使用通用的会话哈希生成逻辑（适用于其他客户端）
 		parsedReq, _ := service.ParseGatewayRequest(body)
+		if parsedReq != nil {
+			parsedReq.SessionContext = &service.SessionContext{
+				ClientIP:  ip.GetClientIP(c),
+				UserAgent: c.GetHeader("User-Agent"),
+				APIKeyID:  apiKey.ID,
+			}
+		}
 		sessionHash = h.gatewayService.GenerateSessionHash(parsedReq)
 	}
 	sessionKey := sessionHash