Add FTPS directory listing regression test

[wfurt]

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.

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