[Tests] Add comprehensive unit tests for os username utility

[gonzaloriestra]

WHY are these changes introduced?

The username utility in packages/cli-kit/src/public/node/os.ts lacked complete test coverage for its various fallback paths (including environment variables, os.userInfo fallbacks, platform-specific command executions via execa, and multiple branch scenarios).

WHAT is this pull request doing?

This PR introduces comprehensive unit tests for the username utility in packages/cli-kit/src/public/node/os.test.ts, covering:

• Correct username resolution when environment variables are present.
• Resolution via os.userInfo() when environment variables are omitted.
• Windows-specific fallback utilizing the whoami command.
• Unix/POSIX fallback using the id -u and id -un commands.
• Handling of name lookup failures and command execution failures.

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 16685993225714546115 started by @gonzaloriestra

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