(GH-1529) Add whatIf support for SSHD config resources

[Gijsreyn]

PR Summary

Add whatIf support for SSHD config resources.

PR Context

Partially addresses #1529.

[Copilot]

Pull request overview

This PR adds --what-if support for the built-in OpenSSH SSHD config resources by wiring a new --what-if flag through the sshdconfig CLI, updating resource manifests to advertise what-if support, and adding Pester coverage to validate both manifest metadata and non-mutating what-if behavior.

Changes:

• Add a --what-if flag to sshdconfig set and plumb it into set handlers to return predicted state without applying changes.
• Update SSHD-related resource manifests to include whatIfArg and whatIfReturns.
• Add Pester tests validating manifest what-if declarations and verifying no file/registry writes during what-if runs.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
resources/sshdconfig/tests/sshdconfig.whatif.tests.ps1	Adds Pester coverage for manifest what-if metadata and what-if “no side effects” behavior.
resources/sshdconfig/sshd-windows.dsc.resource.json	Declares what-if support for the Windows global SSHD settings resource.
resources/sshdconfig/sshd-subsystemList.dsc.resource.json	Declares what-if support for the SubsystemList resource.
resources/sshdconfig/sshd-subsystem.dsc.resource.json	Declares what-if support for the Subsystem resource.
resources/sshdconfig/sshd_config.dsc.resource.json	Declares what-if support for the sshd_config resource.
resources/sshdconfig/src/set.rs	Implements what-if behavior in set paths and returns predicted state when requested.
resources/sshdconfig/src/main.rs	Wires the what_if flag from CLI parsing into invoke_set.
resources/sshdconfig/src/args.rs	Adds --what-if/-w flag to the set subcommand.
resources/sshdconfig/locales/en-us.toml	Adds localized help text for the new what-if flag.

Comments suppressed due to low confidence (1)

resources/sshdconfig/src/set.rs:107

• In --what-if mode, get_existing_config(cmd_info)? can indirectly seed/copy a default sshd_config via ensure_sshd_config_exists(...) (see get_existing_config implementation). That would be a side effect during what-if for repeat-keyword operations when the target config file doesn’t exist yet.

    let (keyword, entry_value) = extract_single_keyword(keyword_input.additional_properties)?;

    let mut existing_config = get_existing_config(cmd_info)?;

[Gijsreyn]

@tgauth, do you mind doing a review on this one?

[tgauth]

LGTM

[Gijsreyn]

@tgauth - newline added and rebased. Ready for merge :)