fix(core/data-include): Validate response.ok before injecting data-include content

[Copilot]

data-include read response.text() without checking the fetch response status, so a server error page (e.g. a 404 HTML page) could be silently injected into the document body.

Changes

• src/core/data-include.js: check response.ok right after fetch(url); if not OK, throw before reading the body so no error-page content ever reaches processResponse.
• Error message: now includes the HTTP status code and status text, plus a troubleshooting hint, surfaced through the existing showError path.
• Tests: added a not-found-include fixture (tests/spec/core/includer.html) pointing at a nonexistent file, and a spec asserting the failed include produces a core/data-include error containing the status code and leaves the target element empty.

const response = await fetch(url);
if (!response.ok) {
  throw new Error(
    `Fetching \`${url}\` failed with status code ${response.status} (${response.statusText}).`
  );
}
const text = await response.text();

• Fixes data-include might insert the 404 error page #5360