Add SFTP tools with SCP fallback

tests/McpSsh.Tests/SftpServiceTests.cs

@@ -0,0 +1,75 @@
+using McpSsh.Server;
+using McpSsh.Server.Audit;
+using McpSsh.Server.Sftp;
+using McpSsh.Server.Ssh;
+using Renci.SshNet;
+
+namespace McpSsh.Tests;
+
+public sealed class SftpServiceTests
+{
+    [Fact]
+    public async Task PutAsync_ReturnsErrorWhenLocalFileIsOutsideWorkingDirectory()
+    {
+        var service = CreateService();
+
+        var result = await service.PutAsync("host", "user", "/tmp/file.txt", "/tmp/file.txt", null, null, null, null, null, CancellationToken.None);
+
+        Assert.Equal("unsafe_local_path", result.Error);
+    }
+
+    [Fact]
+    public async Task PutAsync_ReturnsErrorWhenLocalFileIsMissing()
+    {
+        var service = CreateService();
+
+        var result = await service.PutAsync("host", "user", "missing-file.txt", "/tmp/file.txt", null, null, null, null, null, CancellationToken.None);
+
+        Assert.Equal("local_file_not_found", result.Error);
+    }
+
+    [Fact]
+    public async Task GetAsync_ReturnsErrorWhenLocalFileExistsAndOverwriteIsFalse()
+    {
+        var path = Path.Combine("sftp-test-existing.txt");
+        await File.WriteAllTextAsync(path, "existing");
+        try
+        {
+            var service = CreateService();
+
+            var result = await service.GetAsync("host", "user", "/tmp/file.txt", path, null, null, null, overwrite: false, maxBytes: null, CancellationToken.None);
+
+            Assert.Equal("local_file_exists", result.Error);
+        }
+        finally
+        {
+            File.Delete(path);
+        }
+    }
+
+    private static SftpService CreateService()
+    {
+        return new SftpService(new ThrowingClientFactory(), new CapturingAuditLogger(), new FixedClock());
+    }
+
+    private sealed class ThrowingClientFactory : ISshClientFactory
+    {
+        public SshClient CreateSshClient(SshConnectionRequest request) => throw new NotSupportedException();
+
+        public SftpClient CreateSftpClient(SshConnectionRequest request) => throw new NotSupportedException();
+
+        public ScpClient CreateScpClient(SshConnectionRequest request) => throw new NotSupportedException();
+    }
+
+    private sealed class CapturingAuditLogger : IAuditLogger
+    {
+        public void Log(AuditEvent auditEvent)
+        {
+        }
+    }
+
+    private sealed class FixedClock : ISystemClock
+    {
+        public DateTimeOffset UtcNow => DateTimeOffset.Parse("2026-05-24T12:00:00Z");
+    }
+}