fix(theme-mermaid): re-render diagrams when tabs become visible

[saudademjj]

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

Closes #8357.

Mermaid diagrams inside non-lazy tabs can render while their tabpanel is still hidden. When that happens, opening the second tab can show an incomplete diagram with missing arrows.

This PR re-renders a Mermaid diagram when its surrounding tabpanel becomes visible, and extracts the existing tab-visibility observer into a shared hook so the behavior can be reused outside code blocks.

Test Plan

• added a regression test for a Mermaid block rendered inside a hidden tabpanel that becomes visible
• ran yarn test packages/docusaurus-theme-mermaid/src/theme/Mermaid/__tests__/index.test.tsx --runInBand
• ran yarn build:website:fast

Test links

• Existing dogfooding repro: https://github.com/facebook/docusaurus/blob/main/website/_dogfooding/_pages%20tests/diagrams.mdx#L316
• Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

• Mermaid in tabs. The arrows are not displayed in the second tab #8357

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