refactor(index): replace errorHandled flag with once() listeners

[Fdawgs]

Cleaner than the errorHandled boolean and still rejects if an abort signal is passed.
Functionally the same.

Checklist

• Run npm test
• Commit message adheres to the Conventional commits style, following the @commitlint/config-conventional config

[Copilot]

Pull request overview

This PR refactors the UnRTF#convert child-process event handling in src/index.js by removing the errorHandled boolean and switching to once() event listeners for "error" and "close".

Changes:

• Removed the errorHandled flag used to suppress "close" handling after an "error" event.
• Replaced .on("error"/"close") handlers with .once("error"/"close") listeners.

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