[Refactor] Use lookup map for architecture in platformAndArch

[gonzaloriestra]

WHY are these changes introduced?

To improve readability and maintainability of the platformAndArch utility.

WHAT is this pull request doing?

Replaces an imperative if/else block in platformAndArch with a declarative constant lookup map and nullish coalescing.

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 5490332403927736276 started by @gonzaloriestra

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