fix(theme-classic): support nested elements inside inline code

[MasterIfeanyi]

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

When using CommonMark format, wrapping inline content that contains nested HTML elements (like <var>) inside a <code> tag incorrectly renders as a block code element instead of inline code.

For example, <code>inline <var>var</var></code> was being rendered as a <pre> block instead of staying inline.

This fixes #10533.

Test Plan

The shouldBeInline function in MDXComponents/Code.tsx previously only treated plain strings as inline-safe. This PR introduces a containsNewline helper that recursively checks nested React elements for newlines, so elements like <var> inside <code> are correctly treated as inline as long as they contain no newlines.

To verify: add This is <code>inline <var>var</var></code> to any Markdown file with CommonMark enabled and confirm it renders inline instead of as a code block.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

Closes #10533

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