Add notFound() and NotFoundError to programmatically trigger 404 responses - #17967

Changes

• Adds notFound() and NotFoundError to virtual module astro:navigation to programmatically trigger 404 responses.
• Allows external data loaders (e.g. loadBlog()), API routes, middleware, and .astro components to encapsulate 404 handling without repeating if (!item) return Astro.redirect('/404') checks across page frontmatters.
• Intercepts NotFoundError in the SSR request pipeline (core/routing, core/pages, and core/middleware) so that calling notFound() gracefully renders 404.astro (with dynamic error props) while suppressing fatal 500 server crash logs.

Example

// src/lib/blog.ts
import { notFound } from 'astro:navigation';

export async function loadBlog(id: string) {
  const blog = await fetchBlog(id);
  if (!blog) {
    notFound('Blog post not found'); // Halts execution and triggers 404
  }
  return blog; // TypeScript automatically knows blog is defined!
}

---
// src/pages/blog/[id].astro
import { loadBlog } from '../../lib/blog';

// No need for repetitive `if (!blog) return Astro.redirect('/404')` checks
const blog = await loadBlog(Astro.params.id);
---
<h1>{blog.title}</h1>

Testing

• Added integration tests in packages/astro/test/astro-not-found.test.ts covering .astro frontmatter, dynamic error props, API routes, middleware, standard 500 error propagation, and dev mode.
• Added compile-time type tests in packages/astro/test/types/astro-navigation-virtual-module.ts verifying that notFound() returns never and NotFoundError has the expected prototype and properties.

Docs

• Docs PR will be opened in withastro/docs to document the astro:navigation module and notFound() helper.