Update LLM route handling and improve URL validation

### Summary of Changes
- Set `revalidate` to false for statically generated page content paths in `route.ts`.
- Updated the redirect URL in the LLM route to point to `/llms.txt` instead of the root.
- Enhanced URL validation in `llms.txt` to ensure proper prefixes for documentation and API pages, throwing errors for unexpected URLs.

These changes improve the routing logic and ensure better handling of page URLs.

docs/src/app/llms.mdx/[[...slug]]/route.ts

@@ -19,7 +19,7 @@ export async function GET(
   }
 
   if (!page) {
-    return NextResponse.redirect(new URL('/', request.url), 307);
+    return NextResponse.redirect(new URL('/llms.txt', request.url), 307);
   }
 
   try {

docs/src/app/llms.txt/route.ts

@@ -12,14 +12,20 @@ export async function GET(request: Request) {
   const apiBaseUrl = `${origin}/llms/api/`;
 
   for (const page of source.getPages()) {
-    const relativeUrl = page.url.replace(/^\/docs\/?/, '');
+    if (page.url !== '/docs' && !page.url.startsWith('/docs/')) {
+      throw new Error(`Unexpected page URL "${page.url}" in docs source — expected "/docs" or "/docs/..." prefix`);
+    }
+    const relativeUrl = page.url === '/docs' ? '' : page.url.slice('/docs/'.length);
     if (relativeUrl !== '') {
       docsUrls.add(relativeUrl);
     }
   }
 
   for (const page of apiSource.getPages()) {
-    const relativeUrl = page.url.replace(/^\/api\/?/, '');
+    if (page.url !== '/api' && !page.url.startsWith('/api/')) {
+      throw new Error(`Unexpected page URL "${page.url}" in API source — expected "/api" or "/api/..." prefix`);
+    }
+    const relativeUrl = page.url === '/api' ? '' : page.url.slice('/api/'.length);
     if (relativeUrl !== '') {
       apiUrls.add(relativeUrl);
     }

docs/src/app/llms/[[...slug]]/route.ts

@@ -2,6 +2,8 @@ import { NextResponse, type NextRequest } from 'next/server';
 import { getLLMText } from '../../../../lib/get-llm-text';
 import { apiSource, source } from '../../../../lib/source';
 
+// revalidate = false applies only to the statically generated page content paths
+// (those emitted by generateStaticParams). The empty-slug redirect is always dynamic.
 export const revalidate = false;
 
 function resolvePage(slug: string[] | undefined) {