fix(ci): upgrade Go to 1.26.2 for security fixes, fix flaky WS test

- Upgrade Go from 1.26.1 to 1.26.2 in go.mod and Dockerfile to fix
  govulncheck findings (GO-2026-4947, GO-2026-4946, GO-2026-4870, etc.)
- Fix flaky PassthroughModeRelaysByCaddyAdapter test: tolerate
  StatusNormalClosure error from server after client closes connection

Dockerfile

@@ -7,7 +7,7 @@
 # =============================================================================
 
 ARG NODE_IMAGE=node:24-alpine
-ARG GOLANG_IMAGE=golang:1.26.1-alpine
+ARG GOLANG_IMAGE=golang:1.26.2-alpine
 ARG ALPINE_IMAGE=alpine:3.21
 ARG POSTGRES_IMAGE=postgres:18-alpine
 ARG GOPROXY=https://goproxy.cn,direct

backend/go.mod

@@ -1,6 +1,6 @@
 module github.com/Wei-Shaw/sub2api
 
-go 1.26.1
+go 1.26.2
 
 require (
 	entgo.io/ent v0.14.5

backend/internal/service/openai_ws_forwarder_ingress_session_test.go

@@ -413,7 +413,12 @@ func TestOpenAIGatewayService_ProxyResponsesWebSocketFromClient_PassthroughModeR
 
 	select {
 	case serverErr := <-serverErrCh:
-		require.NoError(t, serverErr)
+		// After normal client close, the server goroutine may receive the close frame
+		// as an error — this is expected behavior, not a test failure.
+		if serverErr != nil {
+			require.Contains(t, serverErr.Error(), "StatusNormalClosure",
+				"server error should only be a normal close frame, got: %v", serverErr)
+		}
 	case <-time.After(5 * time.Second):
 		t.Fatal("等待 passthrough websocket 结束超时")
 	}