fix(core): say why a never-acknowledged subscriptions/listen ended

A modern-era `subscriptions/listen` is a long-lived request: the first
message on its stream must be `notifications/subscriptions/acknowledged`,
and the JSON-RPC `result` for the listen id is reserved as the
graceful-closure marker. A server that answers the listen request with a
bare `result` is saying "acknowledged and closed in the same breath" — the
SDK settles the subscription `graceful` and rejects the pending `listen()`.

The Inspector read that rejection as an ordinary drop: eight re-lists over
roughly a minute, the badge flickering `Reconnecting…`, and a final bare
`Stream ended` that said nothing about why. Accepting the message was
never wrong — it is a well-formed graceful-closure response. The silence
was.

- `core/mcp/subscriptionAck.ts` owns the predicate. The SDK gives this
  close no code of its own — it is the same `SdkError(ConnectionClosed)`
  any pre-ack close carries — so the predicate reads the message, since
  the alternative is refusing to retry a genuinely transient drop.
- The condition is deterministic, so the stream ends on the first
  occurrence instead of spending the whole backoff run. Both routes are
  covered: the one a subscribe/unsubscribe reaches, and the one a
  reconnect reaches when an acknowledged stream drops and the server has
  since started refusing.
- A `"never-acknowledged"` status of its own, not `"ended"` — `ended`
  means an expected close. It surfaces as the thrown error's message, an
  orange "Not acknowledged" badge, and a notice in the panel.
- `subscriptions-never-acknowledged-http.json` reproduces it. It
  acknowledges the first listen that subscribes to a resource and refuses
  every one after, which is what makes the badge reachable by hand.

Closes #2097

Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/src/load-config.ts

@@ -107,7 +107,11 @@ export interface ConfigFile {
      */
     modern?:
       | boolean
-      | { legacy?: "stateless" | "reject"; injectSpecErrors?: boolean };
+      | {
+          legacy?: "stateless" | "reject";
+          injectSpecErrors?: boolean;
+          neverAcknowledgeSubscriptions?: boolean | "after-first";
+        };
   };
 }