Fix duplicates in listing resources and add condition to manifests

[SteveL-MSFT]

PR Summary

• Add condition to several resource manifests that rely on pwsh in case PS7 isn't installed
• Fix showing duplicate resources that have the same version from showing in dsc resource list
• SSHD Subsystem resources were missing from package so added
• MCP list_dsc_resources was sorting an already sorted list, so removed the duplication

[Copilot]

Pull request overview

This PR tightens resource/extension discovery behavior by gating manifests on tool availability, removes duplicate resources (same type + version) from dsc resource list, and aligns MCP list_dsc_resources output with dsc resource list.

Changes:

• Add condition clauses to multiple resource/extension/adapter manifests to hide entries when required executables (e.g., pwsh, sshd) aren’t present.
• Deduplicate discovered manifests by type + version during discovery to prevent duplicate entries in dsc resource list.
• Update Pester tests and MCP resource listing behavior to reflect the new discovery/listing behavior; include missing SSHD subsystem manifests in packaging.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
resources/sshdconfig/sshd-windows.dsc.resource.json	Adds condition to only surface resource when sshd is available.
resources/sshdconfig/sshd-subsystemList.dsc.resource.json	Adds condition to only surface resource when sshd is available.
resources/sshdconfig/sshd-subsystem.dsc.resource.json	Adds condition to only surface resource when sshd is available.
resources/sshdconfig/sshd_config.dsc.resource.json	Adds condition to only surface resource when sshd is available.
resources/PSScript/psscript.dsc.resource.json	Adds condition so PS7-based resource is hidden if pwsh isn’t installed.
extensions/powershell/powershell.dsc.extension.json	Adds condition so PS7 module discovery is hidden if pwsh isn’t installed.
extensions/appx/appx.dsc.extension.json	Adds condition intended to gate discovery based on PowerShell availability.
adapters/powershell/powershell.dsc.resource.json	Adds condition so adapter is hidden if pwsh isn’t installed.
adapters/powershell/PowerShell_adapter.dsc.resource.json	Adds condition so adapter is hidden if pwsh isn’t installed.
lib/dsc-lib/src/discovery/mod.rs	Reworks list_available aggregation to dedupe manifests by type + version.
dsc/src/mcp/list_dsc_resources.rs	Removes secondary sorting/deduping and returns resource summaries in discovery order.
dsc/tests/dsc_resource_list.tests.ps1	Restricts discovery scope for tag/description tests; adds a duplicate-elimination test.
dsc/tests/dsc_mcp.tests.ps1	Updates MCP list test to match non-unique projection/count semantics.
data.build.json	Ensures SSHD subsystem resource manifests are included in built packages.