fix(lint): 修复 golangci-lint 报错

- 修复 gofmt 格式问题
- 修复 staticcheck SA4031 nil check 问题（只在成功时设置 release 函数）
- 删除未使用的 sortAccountsByPriority 函数

backend/internal/handler/gateway_handler.go

@@ -192,9 +192,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					log.Printf("Account wait queue full: account=%d", account.ID)
 					h.handleStreamingAwareError(c, http.StatusTooManyRequests, "rate_limit_error", "Too many pending requests, please retry later", streamStarted)
 					return
-				}
-				accountWaitRelease = func() {
-					h.concurrencyHelper.DecrementAccountWaitCount(c.Request.Context(), account.ID)
+				} else {
+					// Only set release function if increment succeeded
+					accountWaitRelease = func() {
+						h.concurrencyHelper.DecrementAccountWaitCount(c.Request.Context(), account.ID)
+					}
 				}
 
 				accountReleaseFunc, err = h.concurrencyHelper.AcquireAccountSlotWithWaitTimeout(
@@ -314,9 +316,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				log.Printf("Account wait queue full: account=%d", account.ID)
 				h.handleStreamingAwareError(c, http.StatusTooManyRequests, "rate_limit_error", "Too many pending requests, please retry later", streamStarted)
 				return
-			}
-			accountWaitRelease = func() {
-				h.concurrencyHelper.DecrementAccountWaitCount(c.Request.Context(), account.ID)
+			} else {
+				// Only set release function if increment succeeded
+				accountWaitRelease = func() {
+					h.concurrencyHelper.DecrementAccountWaitCount(c.Request.Context(), account.ID)
+				}
 			}
 
 			accountReleaseFunc, err = h.concurrencyHelper.AcquireAccountSlotWithWaitTimeout(

backend/internal/handler/gemini_v1beta_handler.go

@@ -233,9 +233,11 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				log.Printf("Account wait queue full: account=%d", account.ID)
 				googleError(c, http.StatusTooManyRequests, "Too many pending requests, please retry later")
 				return
-			}
-			accountWaitRelease = func() {
-				geminiConcurrency.DecrementAccountWaitCount(c.Request.Context(), account.ID)
+			} else {
+				// Only set release function if increment succeeded
+				accountWaitRelease = func() {
+					geminiConcurrency.DecrementAccountWaitCount(c.Request.Context(), account.ID)
+				}
 			}
 
 			accountReleaseFunc, err = geminiConcurrency.AcquireAccountSlotWithWaitTimeout(