feat(sitemap): add <lastmod> support with per-page timestamps

[iitzIrFan]

fixes: #4462

---

• Added support for across all sitemap generators using a new formatLastMod() helper (ISO-8601 format).
• Updated sitemap generation to include per-URL lastmod values from the best available source:
  • Frontmatter (updated / date)
  • API fields (updatedAt)
  • Filesystem mtime as fallback
• Extended schemas where needed (e.g., added updated to docs).
• Fixed a URLSearchParams.entries() iterator issue in layout.astro.

---

Tested with custom script which can be used for testing this PR,

const http = require('http');
const https = require('https');

const ports = [4321];

const endpoints = [
  '/sitemap/blogs.xml',
  '/sitemap/docs.xml',
  '/sitemap/blueprints.xml',
  '/sitemap/plugins.xml',
  '/sitemap/misc.xml',
];

const urls = ports.flatMap((p) =>
  endpoints.map((e) => `http://localhost:${p}${e}`)
);

const fetch = (url) =>
  new Promise((resolve) => {
    const client = url.startsWith('https') ? https : http;

    try {
      client
        .get(url, (res) => {
          let data = '';

          res.on('data', (chunk) => (data += chunk));
          res.on('end', () => resolve(data));
        })
        .on('error', () => resolve(null));
    } catch {
      resolve(null);
    }
  });

(async () => {
  for (const url of urls) {
    const response = await fetch(url);

    if (response === null) {
      console.error('ERROR', url);
      continue;
    }

    const matches = response.match(/<lastmod>.*?<\/lastmod>/g) || [];
    const unique = [...new Set(matches)];

    console.log(
      url,
      `count=${matches.length}`,
      `unique=${unique.length}`
    );

    if (unique.length > 0) {
      console.log(' sample:', unique.slice(0, 3).join(' | '));
    }
  }
})();

---

ref. #4462 (comment)

[MartinRst]

cc @Piyush-r-bhaskar ok for you to merge this one?

[elevatebart]

Let us check first if it actually works. I believe it does not

[elevatebart]

I do not think this will work on cloudflare.