[Refactor] Memoize username function

[gonzaloriestra]

WHY are these changes introduced?

The username function in @shopify/cli-kit is used in several places, including during app initialization. Since the username is unlikely to change during the execution of a CLI command, memoizing it avoids repeated system calls (environment variable lookups, os.userInfo(), or executing whoami/id).

WHAT is this pull request doing?

• Memoizes the result of the username function in packages/cli-kit/src/public/node/os.ts using a module-level promise.
• Exports a _resetUsernameCache function for test isolation.
• Adds unit tests in packages/cli-kit/src/public/node/os.test.ts to verify memoization and cache reset behavior.

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 4653242786229501269 started by @gonzaloriestra

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[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/public/node/os.d.ts

@@ -3,6 +3,10 @@
  * @returns The username of the current user.
  */
 export declare function username(platform?: typeof process.platform): Promise<string | null>;
+/**
+ * Resets the username cache.
+ */
+export declare function _resetUsernameCache(): void;
 type PlatformArch = Exclude<typeof process.arch, 'x64' | 'ia32'> | 'amd64' | '386';
 type PlatformStrings = Exclude<typeof process.platform, 'win32'> | 'windows';
 /**