Add What-If support for deployment stacks

[Copilot]

Deployment stacks did not support the whatIf operation, unlike regular deployments. Users had no way to preview stack changes before applying them, or to conditionally gate a create operation on whether changes were actually detected.

SDK upgrade

• Bump @azure/arm-resourcesdeploymentstacks to ^2.0.0, which introduces the deploymentStacksWhatIfResultsAt{ResourceGroup,Subscription,ManagementGroup} operation groups needed for stack what-if. Existing stack operations continue to use the same (now-deprecated but still functional) method names, so no other call sites needed changes.

Config & inputs

• Added whatIf as a valid operation value for deploymentStack config (previously only create, validate, delete).

What-if execution

• Added stackWhatIf in stacks.ts, which computes the deployment stack ARM resource ID from the configured scope/name, creates a transient DeploymentStacksWhatIfResult resource (with a minimal 1-day retention interval) to trigger the computation, then fetches the result with full property-level changes.

Presentation & output

• Added formatting logic for DeploymentStacksWhatIfChange results (change legend, stack-level changes, per-resource changes, property diffs, summary), modeled after the Azure CLI's stack what-if formatter.
• Wired the whatIf operation into handler.ts, logging the formatted result and diagnostics.
• Added a hasChanges action output, set for both deployment and deployment stack whatIf operations, so subsequent workflow steps can conditionally decide whether to run create:

- id: whatif
  uses: azure/bicep-deploy@v2
  with:
    type: deploymentStack
    operation: whatIf
    ...

- if: steps.whatif.outputs.hasChanges == 'true'
  uses: azure/bicep-deploy@v2
  with:
    type: deploymentStack
    operation: create
    ...

Docs

• Updated action.yml and README to document the new whatIf operation for deployment stacks and the hasChanges output.

• Fixes [feature] support azure deployment stack whatif #323

[Anthony Martin (anthony-c-martin)]

Copilot please address my comments

[Anthony Martin (anthony-c-martin)]

Why the need to add all these extra methods? Aren't there existing ones we can use?

[Anthony Martin (anthony-c-martin)]

Copilot pls address

[Copilot]

Removed insert/insertLine/getCurrentIndex in d997c64. The only caller (stackWhatIf.ts's conditional stack-changes title) now builds that section into a temporary ColorStringBuilder, then appends the title + built content to the main builder with the existing appendLine/append methods only if something was actually produced — no more inserting at a previously-recorded position.