fix(plex): validate a failover candidate before switching the live endpoint

A transient GET failure on a healthy endpoint could park the client on an
unreachable fallback (e.g. the server host's Docker bridge gateway, which
plex.tv advertises as a local connection) for a full connect timeout, failing
every request in flight during that window (log bbr90).

The cascade now probes each candidate with an unauthenticated /identity
request under the discovery-race budget and only switches when it answers as
the expected server, mirroring the Jellyfin trust gate. Unreachable-looking
private IPv4 candidates stay in the list — a client on the server host can
legitimately reach them, so reachability is probed, not inferred.

test/test_helpers/backend_client_fixtures.dart

@@ -149,6 +149,8 @@ PlexClient testPlexClient({
   http.Client? httpClient,
   Future<http.Response> Function(http.Request request)? handler,
   List<String>? prioritizedEndpoints,
+  http.Client Function()? endpointProbeHttpClientFactory,
+  void Function()? onAllEndpointsExhausted,
   List<({String identifier, String gridEndpoint})> epgProviders = const [],
   String? homeHubKey,
   String? promotedHubKey,
@@ -163,6 +165,8 @@ PlexClient testPlexClient({
     serverName: serverName,
     httpClient: httpClient ?? MockClient(handler ?? _defaultResponse),
     prioritizedEndpoints: prioritizedEndpoints,
+    endpointProbeHttpClientFactory: endpointProbeHttpClientFactory,
+    onAllEndpointsExhausted: onAllEndpointsExhausted,
     epgProviders: epgProviders,
     homeHubKey: homeHubKey,
     promotedHubKey: promotedHubKey,