Declare scoped presence dependency for agent mention picker

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/lib/features/channels/mentions/mention_candidates_provider.dart

@@ -229,7 +229,7 @@ final mentionCandidatesProvider = Provider.family
         history: ref.watch(mentionHistoryProvider)[args.channelId] ?? const {},
         presence: (key) => presence[key] ?? 'unknown',
       );
-    });
+    }, dependencies: [presentedPresenceProvider]);
 
 /// Picker labels compare every selectable choice, before query ranking
 /// filters them, so a row's label does not change as the query narrows.

mobile/test/features/channels/mentions/mention_search_community_test.dart

@@ -1,3 +1,4 @@
+import 'package:buzz/shared/agents/agents_surface.dart';
 import 'dart:async';
 
 import 'package:flutter_test/flutter_test.dart';
@@ -129,6 +130,25 @@ void main() {
     await container.read(channelsProvider.future);
   });
 
+  test('picker supports prototype-scoped remote availability', () async {
+    final scoped = ProviderContainer(
+      parent: container,
+      overrides: [
+        availableRemoteAgentsProvider.overrideWithValue({_alice}),
+      ],
+    );
+    addTearDown(scoped.dispose);
+    final provider = mentionCandidatesProvider((
+      channelId: _channelId,
+      query: '',
+      opening: 2,
+    ));
+    final listener = scoped.listen(provider, (_, _) {});
+    addTearDown(listener.close);
+    await scoped.pump();
+    expect(() => scoped.read(provider), returnsNormally);
+  });
+
   Future<void> passTypingPause() async {
     await Future<void>.delayed(mentionSearchDebounce * 2);
     await container.pump();