feat(theme-live-codeblock): add reset button to live code playground

[NPX2218]

Adds a reset button to live code playgrounds that restores edited code to its original state. The button appears in the playground header alongside the Live Editor label.

Closes #10711

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Live code playgrounds allow users to edit example code interactively, but there was no way to restore the original code without refreshing the entire page. This PR adds a reset button that allows users to easily restore edited code to its initial state, improving the user experience when experimenting with code examples.

As a student at Northeastern University where we use Docusaurus for course documentation, I wanted to contribute to a tool I actively use. This feature would benefit students and developers who learn by experimenting with code examples in live playgrounds.

Test Plan

Tested locally by creating a live code playground and verifying:

1. Editing code in the playground
2. Clicking the Reset button
3. Code restoring to original state

Added dogfooding test page for manual verification.

Test links

• Reset button demo: https://deploy-preview-11675--docusaurus-2.netlify.app/tests/pages/live-playground-test

Related issues/PRs

Closes #10711

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