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/test/java/com/sshmanager/service/SshSessionResizeTest.java

@@ -0,0 +1,27 @@
+package com.sshmanager.service;
+
+import com.jcraft.jsch.ChannelShell;
+import com.jcraft.jsch.Session;
+import org.junit.jupiter.api.Test;
+
+import java.io.InputStream;
+import java.io.OutputStream;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+class SshSessionResizeTest {
+
+    @Test
+    void resizeSetsPtySizeOnChannel() {
+        Session session = mock(Session.class);
+        ChannelShell channel = mock(ChannelShell.class);
+        InputStream out = mock(InputStream.class);
+        OutputStream in = mock(OutputStream.class);
+
+        SshService.SshSession sshSession = new SshService.SshSession(session, channel, out, in);
+        sshSession.resize(120, 40);
+
+        verify(channel).setPtySize(120, 40, 0, 0);
+    }
+}