[Refactor] Use ARCH_MAP lookup in platformAndArch

[gonzaloriestra]

WHY are these changes introduced?

Follows refactoring guidelines to simplify control flow and eliminate imperative conditional logic in favor of a declarative map lookup.

WHAT is this pull request doing?

Refactors platformAndArch in packages/cli-kit/src/public/node/os.ts by:

• Defining a declarative mapping constant ARCH_MAP to map architecture types (e.g., 'x64' -> 'amd64', 'ia32' -> '386').
• Replacing the nested if-else blocks with a lookup from ARCH_MAP using a nullish coalescing fallback (ARCH_MAP[arch] ?? arch).

How to test your changes?

CI

Post-release steps

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 12651267895477029450 started by @gonzaloriestra

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