ssh-add on Windows explicitly forbid host constraint specifications

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest version
• Search the existing issues.

Steps to reproduce

Access to SSH servers that only accept the presentation of a single certificate for authentication is impossible without the ability to specify host constraints.

Although this functionality is properly implemented in the SSH agent, it is explicitly disabled in the Windows version of the ssh-add command.

References:

• https://github.com/PowerShell/openssh-portable/blob/9a43b12c3d1dcb9b742d6b65e5f23d8022017a62/ssh-add.c#L876
• https://github.com/PowerShell/openssh-portable/blob/9a43b12c3d1dcb9b742d6b65e5f23d8022017a62/ssh-add.c#L896

Expected behavior

Enable the feature

Actual behavior

Feature disabled

Error details

Environment data

Name                           Value
----                           -----
PSVersion                      5.1.26100.9444
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.26100.9444
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Version

OpenSSH_for_Windows_9.5p2, LibreSSL 3.8.2 / OpenSSH_for_Windows_10.0p2 Win32-OpenSSH-GitHub, LibreSSL 4.2.0

Visuals

No response

Start in ssh-add.c at the referenced lines 876 and 896, where host constraint specifications are explicitly disabled for Windows. Compare this path with the agent functionality described in the issue and verify ssh-add can accept host constraints on Windows. Done means the command no longer reports "Feature disabled" for this supported use case.