Reproduction for withastro/astro#17092.

On the composable astro/hono path (app.use(middleware()) + app.use(pages())), errors from the Astro middleware layer are not turned into the custom error pages. They escape to the host framework (Hono), which answers with its default plain-text Internal Server Error. Two related gaps:

1. A throw in src/middleware.ts should render the custom 500.astro, but escapes to Hono.
2. An unmatched request, when the custom 404.astro is prerendered (or absent), should render the 404.astro. Instead the middleware layer throws on the missing route and escapes to Hono.

This is the middleware counterpart of withastro/astro#16952 (render-time errors in pages()), which was fixed in #17041. That PR fixed the pages() side and called the middleware case out as out of scope. The all-in-one astro() handler renders both error pages correctly, so the gap is specific to composing middleware() separately.

• / - returns 200.
• /page-throws - throws during page render. With #17041 this renders the custom 500.astro, which confirms the pages() fix is present.
• /boom - a real page, but src/middleware.ts throws before it renders. Should render 500.astro; instead returns Hono's plain-text Internal Server Error (gap 1).
• /does-not-exist - an unmatched route. src/pages/404.astro is prerendered, so the request has no routeData and the middleware layer throws on the missing route. Should render the custom 404.astro; instead returns Hono's plain-text Internal Server Error (gap 2).

npm install
npm run build
npm start            # node ./dist/server/entry.mjs, listens on http://localhost:4321

Then:

curl -i http://localhost:4321/                # 200
curl -i http://localhost:4321/page-throws     # 500, custom 500.astro  (fixed by #17041)
curl -i http://localhost:4321/boom            # 500, Hono "Internal Server Error"  (gap 1)
curl -i http://localhost:4321/does-not-exist  # 500, Hono "Internal Server Error"  (gap 2)

Swapping src/fetch.ts to the all-in-one astro() handler renders both custom error pages for the same requests (/boom -> 500.astro, /does-not-exist -> 404.astro):

import { Hono } from 'hono';
import { astro } from 'astro/hono';

const app = new Hono();
app.use(astro());
export default app;

So middleware errors and unmatched routes can be handled; the gap is specific to composing middleware() separately.

Both middleware gaps (/boom, /does-not-exist) reproduce on any current version, including the public astro@7.0.0-beta.4: the composable middleware() path has no error handling, so the errors escape to Hono.

#17041 only changed the pages() path, so to also see that page render errors are now handled (/page-throws serving 500.astro) while the middleware gaps remain, use a build that includes #17041: main, or the first beta published after 7.0.0-beta.4. On 7.0.0-beta.4 all three error routes hit Hono's default 500.