[Refactor] Use uniq helper in AppManagementClient

[gonzaloriestra]

WHY are these changes introduced?

Following the repository's preference for using shared utility functions over manual implementations for common operations like array deduplication.

WHAT is this pull request doing?

Refactored packages/app/src/cli/utilities/developer-platform-client/app-management-client.ts to replace manual Set deduplication with the uniq helper from @shopify/cli-kit/common/array and use .flatMap() for cleaner nested array flattening.

How to test your changes?

CI

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

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PR created automatically by Jules for task 2931833342598507428 started by @gonzaloriestra

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