feat(gen2-migration): add deployment status validation and unit tests for safe migrations

[sai-ray]

Description of changes

This PR implements deployment status validation to ensure CloudFormation root stack for amplify app is in a stable state before proceeding with Gen 1 to Gen 2 migrations. The implementation adds a validateDeploymentStatus() method that queries the CloudFormation root stack status using AWS SDK v3 and validates that the stack exists and is in either CREATE_COMPLETE or UPDATE_COMPLETE state. When validation fails, it throws appropriate AmplifyError instances (StackNotFoundError or StackStateError) with clear messaging to guide users on resolution steps.

Issue #, if available

Description of how you validated changes

Added unit tests covering multiple scenarios:

• Missing stack validation: Stack name not found in amplify-meta
• Stack not found in CloudFormation: Stack doesn't exist in AWS
• Valid stable states: CREATE_COMPLETE and UPDATE_COMPLETE statuses pass validation
• Invalid states: UPDATE_IN_PROGRESS and ROLLBACK_COMPLETE statuses throw errors

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

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