fix: open the modern listen stream whenever the filter is non-empty (#1920)

On the modern (2026-07-28) era every server→client notification rides the
`subscriptions/listen` stream, but the Inspector only ever opened that stream
from a resource-subscribe click. A tools-only server advertising
`tools.listChanged` therefore had no path to an open stream, so
`notifications/tools/list_changed` could never arrive — even though
`buildSubscriptionFilter()` already modelled the opt-in.

The trigger now matches the filter it builds: the stream opens (and stays open,
and reconnects) whenever the built filter carries anything — subscribed URIs or
an enabled list-change opt-in the server advertises. That mirrors the SDK's own
`ClientOptions.listChanged` auto-open, which opens on a non-empty config ∩
capability intersection. `connect()` opens it once the handlers are registered;
a failure there is handed to the reconnect machinery rather than failing the
connect.

`ResourceSubscriptionStreamState.active` deliberately keeps its narrower
meaning — "at least one URI is subscribed" — because it drives the Subscriptions
section's badge, which has nothing to report for a listChanged-only stream. That
also preserves the invariant the rest of the file is written against: an empty
subscribed set is never announced alongside an `active` stream.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HDdo1rNRVQnVRRdsqSrvG3

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

@@ -52,7 +52,9 @@ export interface ResourceControlsProps {
    * Modern-era `subscriptions/listen` stream state (#1630). When `active`
    * (modern era with at least one subscription) the Subscriptions section shows
    * a stream-status badge in its panel and a status dot in its header. Legacy
-   * connections pass `active: false` (or omit it) and see neither.
+   * connections pass `active: false` (or omit it) and see neither — and so does
+   * a stream open purely for list-change notifications, which this section has
+   * nothing to say about (#1920).
    */
   subscriptionStreamState?: ResourceSubscriptionStreamState;
   /** Negotiated protocol era; gates the modern subscription stream chrome. */