NoFallbackError: replace internal-jargon message with actionable guidance

[yogeshwaran-c]

What?

Replace the NoFallbackError message from the unhelpful "Internal: NoFallbackError" with one that names the most common cause and links to the relevant docs.

Why?

NoFallbackError is an internal control-flow signal — the framework normally catches it and turns it into a 404 response, so the message is rarely seen by end users. But when it does leak to a server log (as reproduced in #87738 — a parent app router segment with dynamicParams: false plus a request whose params aren't in generateStaticParams()), the user sees:

```
Error: Internal: NoFallbackError
at (.next/server/chunks/ssr/babdf1ac..js:2:1128)
...
```

…which gives them no idea what went wrong, what to look for in their app, or where to read more. They have to chase it through Next.js source code to figure out the connection to dynamicParams.

How?

Replace the static message with one that names the common cause and links to the route segment config docs:

```
No fallback was rendered for this route, so the request returned a 404.
A common cause is dynamicParams: false on a parent segment combined with
a request whose params were not returned by generateStaticParams().
See https://nextjs.org/docs/app/api-reference/file-conventions/route-segment-config#dynamicparams
```

Also switched from super(); this.message = ... (the old form) to the conventional super(message).

The error message is the only public surface of the class — no behavior change for the framework's normal catch-and-handle flow. Every instanceof NoFallbackError check in base-server.ts, next-server.ts, router-server.ts, etc., still works exactly as before.

Scope

Just the message. The PR deliberately does not try to enrich NoFallbackError with per-call context (which segment / route / params triggered it). That would require plumbing through every throw site (see grep for new NoFallbackError() in app-page.ts, app-route.ts, pages-handler.ts) and is a separate, larger change. The static message is already a substantial improvement on the no-info-at-all status quo.

Test plan

Added test/unit/no-fallback-error.test.ts with two cases:

• The class is an Error subclass (sanity check).
• The message contains dynamicParams, generateStaticParams, and a nextjs.org/docs/ URL — locks in the actionable wording so a future edit can't silently regress to "Internal: NoFallbackError".

npx jest test/unit/no-fallback-error.test.ts → 2 passed.

Fixes #87738

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