feat: refine sftp pane upload workflow

backend/src/test/java/com/sshmanager/service/SftpServiceTest.java

@@ -1,5 +1,9 @@
 package com.sshmanager.service;
 
+import com.jcraft.jsch.ChannelSftp;
+import com.jcraft.jsch.Session;
+import com.jcraft.jsch.SftpATTRS;
+import com.jcraft.jsch.SftpException;
 import com.sshmanager.entity.Connection;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -8,6 +12,8 @@ import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 
 import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 class SftpServiceTest {
 
@@ -48,4 +54,29 @@ class SftpServiceTest {
         executorService.shutdown();
         assertTrue(executorService.isTerminated() || executorService.isShutdown());
     }
+
+    @Test
+    void statIfExistsReturnsNullWhenRemotePathIsMissing() throws Exception {
+        Session session = mock(Session.class);
+        ChannelSftp channel = mock(ChannelSftp.class);
+        when(channel.stat("/missing.txt")).thenThrow(new SftpException(ChannelSftp.SSH_FX_NO_SUCH_FILE, "missing"));
+
+        SftpService.PathInfo result = sftpService.statIfExists(new SftpService.SftpSession(session, channel), "/missing.txt");
+
+        assertNull(result);
+    }
+
+    @Test
+    void statIfExistsReturnsDirectoryFlagForExistingPath() throws Exception {
+        Session session = mock(Session.class);
+        ChannelSftp channel = mock(ChannelSftp.class);
+        SftpATTRS attrs = mock(SftpATTRS.class);
+        when(channel.stat("/existing")).thenReturn(attrs);
+        when(attrs.isDir()).thenReturn(true);
+
+        SftpService.PathInfo result = sftpService.statIfExists(new SftpService.SftpSession(session, channel), "/existing");
+
+        assertNotNull(result);
+        assertTrue(result.directory);
+    }
 }