Fix reason validation on adapter

[Gijsreyn]

PR Summary

Fixed the adapter to properly display validation error reasons when operations fail on adapted resources.

PR Context

Fix #1337.

[Gijsreyn]

@SteveL-MSFT - if you have some time to review, we discussed this during WG that this was missing for adapters, thus we could pull it in. If it's of course good :)

[Copilot]

Pull request overview

Fixes adapter-backed resource validation failures so that when an adapter’s validate operation returns valid: false with a custom reason, DSC surfaces that reason instead of a generic message (addresses #1337).

Changes:

• Use ValidateResult.reason (when present) as the DscError::Validation message for command-resource JSON verification failures.
• Add a PowerShell adapter integration test asserting the custom validation reason appears in error output.
• Update the TestAdapter Validate operation to emit valid: false plus a reason for a specific test case.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
lib/dsc-lib/src/dscresources/command_resource.rs	Propagates adapter/command validate failure reason into returned validation errors.
adapters/powershell/Tests/powershellgroup.resource.tests.ps1	Adds a regression test that expects the adapter-provided validation reason to be printed.
adapters/powershell/Tests/TestAdapter/testadapter.resource.ps1	Adds a validation-failure path that returns a custom reason for test coverage.

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