fix(core): render base URL suggestion as text (AI-assisted)

[ishaanlabs-gg]

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.
• This is not a new API or substantial change.

Motivation

(AI-assisted)

Closes #12216.

The base URL issue banner currently writes the suggested baseUrl with innerHTML, even though the value is intended to be displayed as plain text from window.location.pathname.

Using textContent keeps the rendered output the same for normal paths while avoiding an unnecessary HTML sink in the inline banner script.

Test Plan

(AI-assisted)

• Added a regression test that checks the suggested baseUrl assignment uses textContent and does not use innerHTML.
• Verified the source and regression test with a focused Node check.
• git diff --check HEAD~1 HEAD -- packages/docusaurus/src/client/BaseUrlIssueBanner/index.tsx packages/docusaurus/src/client/__tests__/BaseUrlIssueBanner.test.ts
• npm exec --yes --package oxfmt@0.47.0 --call "oxfmt --check packages/docusaurus/src/client/BaseUrlIssueBanner/index.tsx packages/docusaurus/src/client/__tests__/BaseUrlIssueBanner.test.ts"

Test links

Deploy preview: https://deploy-preview-12220--docusaurus-2.netlify.app/

Related issues/PRs

Closes #12216.

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