fix(core): use locale url in site config

[rohan-patnaik]

Fixes #12013

Summary

This copies the active locale URL into siteConfig when loading a localized site.

Why

Docusaurus already applies the locale-specific baseUrl, but kept the top-level url. Plugins that receive siteConfig, including the sitemap plugin, could therefore generate URLs with the primary locale's domain even when the current locale is configured for a separate domain.

Test plan

• nvm use 26 && yarn oxfmt --list-different packages/docusaurus/src/server/site.ts packages/docusaurus/src/server/__tests__/site.test.ts
• nvm use 26 && yarn eslint --cache --report-unused-disable-directives packages/docusaurus/src/server/site.ts packages/docusaurus/src/server/__tests__/site.test.ts
• nvm use 26 && yarn test packages/docusaurus/src/server/__tests__/site.test.ts currently blocked locally because this checkout needs additional built workspace packages (@docusaurus/bundler, then @docusaurus/faster/@docusaurus/babel) after installing dependencies with scripts disabled.

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[slorber]

LGTM 👍

Thanks

Although our e2e tests do not cover this well, I tested it locally and it seems like a reasonable thing to do

[rohan-patnaik]

Thanks for the review.
Appreciate the quick merge.