feat: Adding render preview for expansion

[yhakbar]

Description

Adds render expansion previews as comments below HCL blocks with expansion blocks.

As part of adding this, I realized we never landed the plane on duplicate dependency labels, as discussed in #4504. This PR adds the duplicate-dependency-labels strict control, which warns by default and errors when enabled if duplicate dependency labels are discovered.

TODOs

Read the Gruntwork contribution guidelines.

• I authored this code entirely myself
• I am submitting code based on open source software (e.g. MIT, MPL-2.0, Apache)
• I am adding or upgrading a dependency or adapted code and confirm it has a compatible open source license
• Update the docs.
• Update the changelog in the docs.
• Run the relevant tests successfully, including pre-commit checks.
• This change is backwards compatible.
• If this change is not forwards compatible (e.g. a new feature), it is gated behind a feature flag.

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Project	Deployment	Actions	Updated (UTC)
terragrunt-docs	Ready	Preview	Aug 25, 2026 8:56pm

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