fix(network): 修复长耗时请求被误报「Network disconnected」中断的问题

客户端扩展在慢请求（如提交信息生成）开始约 10 秒后会探测
NetworkService/IsConnected，非 200 应答即判定断网并中止进行中的请求。
此前该探测落入通配路由返回 404，导致模型仍在正常返回时界面先报断连。

- host.go: 注册 IsConnected 精确路由，恒定以 200 + 空 IsConnectedResponse 应答
- client.go: newProtoMessage 补齐 aiserver.v1.IsConnectedResponse 类型，
  避免 mock 编码失败导致探测返回 502
- state_db.go: 新增强开 statsig gate 机制（cursorStateEnabledStatsigGates），
  强制启用 disable_network_change_monitor_local 从源头关闭该探测；
  disableCursorStatsigGates 泛化为 syncCursorStatsigGateOverrides

internal/backend/host.go

@@ -691,6 +691,20 @@ func (host *Host) rebuildLocked(cfg serverconfig.Config) error {
 				return nil
 			}),
 		),
+		// The always-local extension probes NetworkService/IsConnected roughly 10s
+		// after any slow request starts. A 404 here is treated as "network
+		// disconnected" and aborts in-flight work (e.g. commit message
+		// generation) even while the model is still streaming. Always answer OK.
+		server.POST("/aiserver.v1.NetworkService/IsConnected",
+			server.Name("network_is_connected"),
+			server.ConnectUnary(),
+			server.Local(upstream.MockProtoAction(routeDeps, upstream.CompatRouteConfig{
+				Name:          "network_is_connected",
+				StatusCode:    http.StatusOK,
+				MockProtoType: "aiserver.v1.IsConnectedResponse",
+				MockBuilder:   upstream.EmptyMockBuilder,
+			})),
+		),
 		server.Any("/aiserver.v1.NetworkService/*",
 			server.Name("network_service"),
 			server.HTTP(),