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>

clients/web/src/components/groups/ResourceControls/ResourceControls.test.tsx

@@ -6,6 +6,7 @@ import type {
   ResourceTemplateType as ResourceTemplate,
 } from "@modelcontextprotocol/client";
 import type { InspectorResourceSubscription } from "@inspector/core/mcp/types.js";
+import { NEVER_ACKNOWLEDGED_SUBSCRIPTION_MESSAGE } from "@inspector/core/mcp/subscriptionAck.js";
 import { renderWithMantine, screen } from "../../../test/renderWithMantine";
 import {
   ResourceControls,
@@ -394,6 +395,37 @@ describe("ResourceControls", () => {
       expect(screen.getByText("Subscriptions (0)")).toBeInTheDocument();
       expect(screen.queryByText("Listening")).not.toBeInTheDocument();
     });
+
+    // A badge tooltip is not enough for this one: the server broke the listen
+    // contract and the user has to be told without hovering (#2097).
+    it("spells out a never-acknowledged close in the panel", () => {
+      renderWithMantine(
+        <ControlledResourceControls
+          protocolEra="modern"
+          subscriptionStreamState={{
+            active: true,
+            status: "never-acknowledged",
+            honoredUris: [],
+          }}
+        />,
+      );
+      expect(screen.getByText("Not acknowledged")).toBeInTheDocument();
+      expect(
+        screen.getByText(NEVER_ACKNOWLEDGED_SUBSCRIPTION_MESSAGE),
+      ).toBeInTheDocument();
+    });
+
+    it("shows no such notice while the stream is healthy", () => {
+      renderWithMantine(
+        <ControlledResourceControls
+          protocolEra="modern"
+          subscriptionStreamState={activeAck}
+        />,
+      );
+      expect(
+        screen.queryByText(NEVER_ACKNOWLEDGED_SUBSCRIPTION_MESSAGE),
+      ).not.toBeInTheDocument();
+    });
   });
 
   // A failed load is rendered above the list instead of leaving the panel

clients/web/src/components/groups/ResourceControls/ResourceControls.tsx

@@ -1,4 +1,12 @@
-import { Accordion, Group, Stack, Text, TextInput, Title } from "@mantine/core";
+import {
+  Accordion,
+  Alert,
+  Group,
+  Stack,
+  Text,
+  TextInput,
+  Title,
+} from "@mantine/core";
 import { ClearButton } from "../../elements/ClearButton/ClearButton";
 import { RiArrowRightSLine } from "react-icons/ri";
 import type {
@@ -10,6 +18,7 @@ import type {
   InspectorResourceSubscription,
   ResourceSubscriptionStreamState,
 } from "../../../../../../core/mcp/types.js";
+import { NEVER_ACKNOWLEDGED_SUBSCRIPTION_MESSAGE } from "../../../../../../core/mcp/subscriptionAck.js";
 import { isModernEra } from "../../elements/EraBadge/eraUtils";
 import { SubscriptionStreamBadge } from "../../elements/SubscriptionStreamBadge/SubscriptionStreamBadge";
 import { ListChangedIndicator } from "../../elements/ListChangedIndicator/ListChangedIndicator";
@@ -33,6 +42,15 @@ const TightRow = Group.withProps({ gap: "xs", wrap: "nowrap" });
 // instead of overflowing the card (#1462).
 const SidebarStack = Stack.withProps({ gap: "sm", flex: 1, mih: 0 });
 
+// The never-acknowledged close (#2097) is the one stream status whose reason has
+// to be readable without hovering the badge: it is a server-conformance problem
+// the user has to act on, not a lifecycle event they can wait out.
+const StreamNotice = Alert.withProps({
+  color: "orange",
+  variant: "light",
+  title: "Subscription not acknowledged",
+});
+
 const SearchInput = TextInput.withProps({
   flex: 1,
   placeholder: "Search...",
@@ -358,6 +376,11 @@ export function ResourceControls({
             </Accordion.Control>
             <Accordion.Panel>
               <Stack gap="xs">
+                {streamStatus === "never-acknowledged" && (
+                  <StreamNotice>
+                    {NEVER_ACKNOWLEDGED_SUBSCRIPTION_MESSAGE}
+                  </StreamNotice>
+                )}
                 {filteredSubscriptions.map((sub) => (
                   <ResourceSubscribedItem
                     key={sub.resource.uri}