Add FTPS directory listing regression test (#135077)

## Summary

- Add enterprise coverage for synchronous `FtpWebRequest`
`ListDirectoryDetails` requests with and without TLS.
- Verify that the returned listing is plaintext, contains the uploaded
fixture, and completes with `FtpStatusCode.ClosingData`.
- Clean up the uploaded fixture even when the listing assertion fails.
- Document the additional FTPS directory-listing coverage.

This directly covers the scenario reported in #134130. The
product fix is already present on `main` via #123234; this PR adds
coverage for the specific listing operation from the newer report.

## Testing

- `./build.sh clr+libs -rc release` in the Linux enterprise test client:
succeeded with 0 warnings and 0 errors.
- `/repo/dotnet.sh build /t:Test
/p:XunitMethodName=System.Net.Tests.FtpWebRequestStreamDisposalTest.FtpListDirectoryDetails_ReturnsPlainText`:
2 passed, 0 failed, 0 skipped.
- Negative control: temporarily restored the old synchronous
stream-selection expression and rebuilt `System.Net.Requests`; the
non-TLS case passed while the TLS case failed in
`StreamReader.ReadToEnd` with `ObjectDisposedException`.
- The attached issue repro's request sequence was also verified against
controlled ProFTPD: .NET 10.0.0 returned TLS ciphertext despite status
226, while .NET 11 RC1 returned the expected plaintext listing.

> [!NOTE]
> This pull request description was generated with GitHub Copilot.

Author: wfurt

src/libraries/System.Net.Requests/tests/EnterpriseTests/FtpWebRequestStreamDisposalTest.cs

@@ -159,6 +159,63 @@ public async Task FtpDownload_StreamDisposal(bool useSsl)
                 _output.WriteLine($"Cleanup failed: {ex.Message}");
             }
         }
+
+        [ConditionalTheory(typeof(EnterpriseTestConfiguration), nameof(EnterpriseTestConfiguration.Enabled))]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task FtpListDirectoryDetails_ReturnsPlainText(bool useSsl)
+        {
+            string fileName = $"test_{Guid.NewGuid()}.txt";
+            string fileUrl = GetFtpUrl(fileName);
+
+            FtpWebRequest uploadRequest = (FtpWebRequest)WebRequest.Create(fileUrl);
+            uploadRequest.Method = WebRequestMethods.Ftp.UploadFile;
+            uploadRequest.EnableSsl = useSsl;
+            uploadRequest.Credentials = EnterpriseTestConfiguration.FtpNetworkCredentials;
+
+            using (Stream requestStream = await uploadRequest.GetRequestStreamAsync())
+            {
+                await requestStream.WriteAsync("test"u8.ToArray());
+            }
+
+            using (FtpWebResponse uploadResponse = (FtpWebResponse)await uploadRequest.GetResponseAsync())
+            {
+                Assert.Equal(FtpStatusCode.ClosingData, uploadResponse.StatusCode);
+            }
+
+            try
+            {
+                FtpWebRequest listRequest = (FtpWebRequest)WebRequest.Create(GetFtpUrl(string.Empty));
+                listRequest.Method = WebRequestMethods.Ftp.ListDirectoryDetails;
+                listRequest.EnableSsl = useSsl;
+                listRequest.Credentials = EnterpriseTestConfiguration.FtpNetworkCredentials;
+
+                using (FtpWebResponse response = (FtpWebResponse)listRequest.GetResponse())
+                using (StreamReader reader = new StreamReader(response.GetResponseStream()))
+                {
+                    string listing = reader.ReadToEnd();
+                    Assert.Contains(fileName, listing);
+                    Assert.Equal(FtpStatusCode.ClosingData, response.StatusCode);
+                }
+            }
+            finally
+            {
+                try
+                {
+                    FtpWebRequest deleteRequest = (FtpWebRequest)WebRequest.Create(fileUrl);
+                    deleteRequest.Method = WebRequestMethods.Ftp.DeleteFile;
+                    deleteRequest.EnableSsl = useSsl;
+                    deleteRequest.Credentials = EnterpriseTestConfiguration.FtpNetworkCredentials;
+
+                    using FtpWebResponse deleteResponse = (FtpWebResponse)await deleteRequest.GetResponseAsync();
+                    Assert.Equal(FtpStatusCode.FileActionOK, deleteResponse.StatusCode);
+                }
+                catch (Exception ex)
+                {
+                    _output.WriteLine($"Cleanup failed: {ex.Message}");
+                }
+            }
+        }
 #pragma warning restore SYSLIB0014
     }
 }

src/libraries/System.Net.Requests/tests/EnterpriseTests/README.md

@@ -11,4 +11,5 @@ These tests validate FTP operations with SSL/TLS (FTPS) against a ProFTPD server
 The tests cover:
 - FTP file upload with explicit SSL/TLS
 - FTP file download with explicit SSL/TLS
+- FTP directory listing with explicit SSL/TLS
 - Proper SSL/TLS stream closure to prevent protocol violations