[Gijs Reijn (Gijsreyn)]

PR Summary

This change:

• Adds the --target ssh://[user@]host[:port] option to run dsc config get, set, test and export on another machine.
• Adds a ssh_subsystem.dsc.yaml configuration that registers dsc server as the dsc SSH subsystem on the target.
• Makes the MCP server report itself as dsc with its version, and adds the equivalent CLI exit code to invoke_dsc_config errors.
• Adds Pester tests for the remote path and extends the server tests.
• Implements the experimental part of the RFC in RFC Draft of SSH subsystem #1803. Relates to DSC as a SSH subsystem #1495.

PR Context

#1495 asks for a way to apply a configuration to a machine without copying files to it or wrapping dsc in a remote shell command. The target already has what's needed: dsc server speaks MCP over standard input and output, and sshd can start it as a subsystem.

With --target, dsc reads the document and parameters on the client, starts the system ssh client with the dsc subsystem, calls the invoke_dsc_config tool once, and prints the result as a local run does. Nothing has to exist on the target except dsc and the subsystem entry.

Remarks on the design

• Uses the system ssh client. Authentication, host keys, jump hosts and ~/.ssh/config aliases work as they do for ssh. DSC handles no credentials. A password in the URI is rejected, and port and user are left to ssh when the URI has none.
• Rejected before connecting. Any command other than the four config operations, --system-root, and a target that isn't a plain ssh:// URI fail as invalid arguments, so an unsupported combination never opens a connection.
• Server version check. The client refuses a server that doesn't identify itself as dsc 3.4.0-preview.3 or newer. Older servers ignore arguments they don't know, so --what-if would apply changes for real.
• Same exit codes as a local run. The server puts the CLI exit code in the error data and the client returns it. A connection or handshake failure returns the new exit code 11, with what ssh wrote on standard error.
• Remote traces. The subsystem runs with --trace-format json. The client emits those traces through its own tracing settings, prefixed with the host.
• Include resolves on the target. A document that uses Microsoft.DSC/Include gets a warning that file paths are resolved on the target, not on the client.
• Size limits. A request over 16 MB is refused on the client, because the server drops it without answering.
• DSC_SSH_EXECUTABLE. Replaces the ssh found on PATH. The tests use it to run against a stand-in client.
• Out of scope. Other transports, dsc resource commands, multiple targets in one run, and reference docs and a changelog entry, which wait on the RFC.