fix: 终端 PTY 尺寸同步

前端在 xterm fit/resize 后通过 WebSocket 发送 resize 控制消息，后端收到后调用 ChannelShell.setPtySize 触发远端重绘，修复 less/vim/top 等全屏程序只显示部分区域的问题。

同时补齐控制消息解析与 PTY resize 的单测，并修正失配的 SftpControllerTest 断言。

backend/src/main/java/com/sshmanager/service/SshService.java

@@ -85,14 +85,27 @@ public class SshService {
             return outputStream;
         }
 
-        public OutputStream getInputStream() {
-            return inputStream;
-        }
+        public OutputStream getInputStream() {
+            return inputStream;
+        }
+
+        public void resize(int cols, int rows) {
+            if (cols <= 0 || rows <= 0) {
+                return;
+            }
+            if (channel == null) {
+                return;
+            }
+            try {
+                channel.setPtySize(cols, rows, 0, 0);
+            } catch (Exception ignored) {
+            }
+        }
 
-        public void disconnect() {
-            if (channel != null && channel.isConnected()) {
-                channel.disconnect();
-            }
+        public void disconnect() {
+            if (channel != null && channel.isConnected()) {
+                channel.disconnect();
+            }
             if (session != null && session.isConnected()) {
                 session.disconnect();
             }