Expand test coverage to match specs

Endpoints/AdminEndpoints.cs

@@ -191,7 +191,7 @@ public static class AdminEndpoints
             var suggestions = await db.Suggestions.ToListAsync();
             var target = suggestions.FirstOrDefault(s => s.Id == request.SuggestionId);
             if (target is null)
-                return Results.NotFound(new { error = "Suggestion not found." });
+                return Results.Ok(new { UnlinkedSuggestionIds = Array.Empty<int>(), UnfinalizedPlayers = 0 });
 
             var rootIndex = EndpointHelpers.BuildLinkRoots(suggestions.Select(s => (s.Id, s.ParentSuggestionId)));
             if (!rootIndex.TryGetValue(target.Id, out var rootId))