Fix: Add missing Chat.js and complete ContactList.js dispatch logic i…

[vaibhavibaware]

This PR addresses two key issues in the Messenger example on the useReducer tutorial page:

1. Adds a missing Chat.js component file, which is referenced in Messenger.js.
2. Completes the dispatch logic in ContactList.js by handling the changed_selection action correctly.

fixes #7898

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[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

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[gaearon]

There's nothing to be fixed here.

The code you're editing is a challenge: https://react.dev/learn/extracting-state-logic-into-a-reducer#dispatch-actions-from-event-handlers

The challenge is intended to be completed by the reader, so the TODOs are for the reader. And as your PR shows, editing the provided files is enough to complete the challenge.

Your PR also does not "add Chat.js file", and that file is not actually missing.