Add format option to paginate() for customizing pagination URLs

[astrobot-houston]

Changes

• Adds a format option to PaginateOptions that accepts a (url: string) => string callback. When provided, it is applied to all pagination URLs (current, next, prev, first, last) after they are constructed.
• This is a non-breaking, opt-in addition — default behavior is completely unchanged. Users deploying to static file servers without URL rewrite rules can now use this to append .html (or apply any other transformation) to pagination URLs generated by paginate().

paginate(items, {
  pageSize: 10,
  format: (url) => `${url}.html`,
})

Closes #13604

Testing

• Added 3 new describe blocks (8 test cases) to packages/astro/test/units/render/paginate.test.ts covering: format applied to all URL properties, format skipped for undefined URLs, format applied after base path is prepended, and no regression when format is omitted.

Docs

• The new format option is documented inline via JSDoc on PaginateOptions. A docs-site update to the paginate() reference may be warranted to surface this option for users hitting 404s on static file servers.

[changeset-bot]

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Latest commit: daa9ae8

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[matthewp]

missing a changeset and needs docs