fix(metadata): preserve URL instance pathname in alternates

[mango766]

What?

Fix resolveAlternateUrl to preserve the full pathname of URL instances passed in metadata.alternates.languages, alternates.canonical, alternates.media, and alternates.types.

Why?

When users pass a URL object for alternate language URLs (e.g. new URL('/nl/blog', 'http://localhost:3000')), the URL's own pathname was being silently discarded. The old code did new URL(pathname, url), treating the user's URL as a base and resolving it against the current page's pathname — effectively replacing /nl/blog with /blog.

This means any locale prefix, subpath, or custom path encoded in the URL was lost. The workaround was to call .toString() on the URL, which shouldn't be necessary.

Reported by multiple users in #68351, confirmed still present as of v15.2.3.

How?

In resolveAlternateUrl, when the input is a URL instance, convert it to its href string before passing it downstream to resolveAbsoluteUrlWithPathname. This way the full path the user specified is treated as an absolute URL and preserved as-is.

Before (broken):

if (url instanceof URL) {
  const newUrl = new URL(pathname, url)  // ← overwrites url's own pathname
  // ...
  url = newUrl
}

After (fixed):

if (url instanceof URL) {
  url = url.href  // ← preserves url's full path
}

Test plan

• Added test/e2e/app-dir/metadata/app/alternates/url-instance/page.tsx fixture that uses URL instances for canonical and languages
• Added test case 'should preserve URL instance pathname in alternates' to the existing metadata e2e test suite
• Existing alternate tests (relative paths, string URLs) continue to pass as before

Fixes #68351

[nextjs-bot]

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