feat(gen2-migration): implement stateful resources validation for decommission command

[sai-ray]

Description of changes

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This PR implements the validate() method for decommission command to ensure decommissioning won't accidentally delete stateful resources. The implementation uses CloudFormation change sets to preview what resources would be deleted during decommissioning without actually performing the deletion. The change set is created with an empty template ( { Resources: {} }), which simulates deleting all stack resources, allowing us to validate the impact before execution.

Flow:

1. Get Stack Name - Retrieves from stateManager.getMeta(). Stack name format is amplify-{appId}-{envName}, which CloudFormation uses as the unique identifier.
2. Create Change Set - Uses Date.now() for unique naming. Empty template simulates deleting all resources.
3. Wait for Completion - waitUntilChangeSetCreateComplete with maxWaitTime: 120 secs (SDK default) polls until CloudFormation computes all changes.
4. Describe & Store - Retrieves change set details into local variable for validation.
5. Cleanup - Deletes change set from AWS.

Lint Suppressions:
@typescript-eslint/no-explicit-any (line 17): validateStatefulResources() doesn't use context, so {} as any is acceptable.
spellcheck/spell-checker (line 19): "Stateful" is valid technical terminology.

Issue #, if available

PR #14297

Description of how you validated changes

Checklist

• PR description included
• Relevant documentation is changed or added (and PR referenced)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.