hive otel: keep file_storage defined when the collector authenticates upstream

The hive collector's `settings` was `{ extensions.file_storage = …; … }
// lib.optionalAttrs senderAuth { extensions.oauth2client/swarm = …; }`.
`//` is shallow, so whenever senderAuth held (every valid hive config)
the right-hand `extensions` replaced the left one and `file_storage`
vanished, while `service.extensions` still listed it. `otelcol validate`
then failed the config.yaml build:

  Error: service::extensions: references extension "file_storage" which is not configured

Broken by 02831fb5e (PR #5178), which added `extensions.file_storage`
to the left-hand attrset; the `//` block for the auth extension predates
it (cb787997bd). Both extensions now live in one `extensions` attrset.

module-eval-hive-otel gains a case that every name in
`service.extensions` is defined under `extensions`; it evaluates false
on the parent commit and true here.

Fixes #5240

nix/module-eval/hive-otel.nix

@@ -111,6 +111,16 @@ let
         !(builtins.elem "deltatocumulative" hiveOtelPipelines.logs.processors)
         && builtins.elem "deltatocumulative" hiveOtelPipelines.metrics.processors;
     }
+    {
+      # `otelcol validate` refuses a `service.extensions` entry with no
+      # definition, and that runs when the unit is built, not in this suite.
+      # `hiveOtel` authenticates upstream, so both extensions are listed;
+      # the length clause keeps the check from passing on an empty list.
+      name = "the hive collector defines every extension its service lists";
+      ok =
+        lib.length hiveOtelSettings.service.extensions == 2
+        && lib.all (e: hiveOtelSettings.extensions ? ${e}) hiveOtelSettings.service.extensions;
+    }
     {
       # The counters that say telemetry is being LOST — refused, failed,
       # queue depth — are served on loopback and reach no store unless