add WhatIfReturns field to resource manifest

[tgauth]

PR Summary

• add WhatIfReturns field to resource manifest
• update dsc test resource to utilize WhatIfReturns (for StateAndDiff scenario)
• update registry resource manifest to use WhatIfArg and WhatIfReturns (for State scenario)

PR Context

• in the process of updating the registry resource manifest to use WhatIfArg, it became clear that there was no way to specify a different return between an Actual and a WhatIf execution for set

[Copilot]

Pull request overview

Adds support for specifying a different set return shape during what-if execution via a new whatIfReturns manifest field, and updates test/registry manifests and tests to exercise state-vs-stateAndDiff scenarios.

Changes:

• Add whatIfReturns to the resource manifest set method and honor it during invoke_set when executionType=WhatIf.
• Extend dsctest what-if output to optionally emit stateAndDiff (two-line output) and add a new test manifest entry.
• Update the Registry resource manifest to rely on whatIfArg/whatIfReturns instead of a dedicated whatIf operation, and add/adjust PowerShell tests.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tools/dsctest/src/whatif.rs	Adds fromResource field to dsctest what-if state payload.
tools/dsctest/src/main.rs	Adds --state-and-diff mode for dsctest what-if output (state + diff).
tools/dsctest/src/args.rs	Adds CLI flag to control stateAndDiff output.
tools/dsctest/dsctest.dsc.manifests.json	Adds Test/WhatIfReturnDiff resource manifest using whatIfReturns.
resources/registry/registry.dsc.resource.json	Migrates Registry manifest to whatIfArg/whatIfReturns and removes dedicated whatIf op.
lib/dsc-lib/src/dscresources/resource_manifest.rs	Introduces whatIfReturns on SetMethod.
lib/dsc-lib/src/dscresources/command_resource.rs	Uses whatIfReturns to select return parsing behavior for what-if set.
dsc/tests/dsc_whatif.tests.ps1	Adds coverage for whatIfReturns=stateAndDiff behavior.
dsc/tests/dsc_resource_set.tests.ps1	Adjusts assertion for changedProperties shape in what-if set test.

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