Fix Galaxy paging review findings

src/MxGateway.Server/Galaxy/GalaxyHierarchyCache.cs

@@ -99,6 +99,14 @@ public sealed class GalaxyHierarchyCache : IGalaxyHierarchyCache
                     LastQueriedAt = queriedAt,
                     LastSuccessAt = queriedAt,
                     LastError = null,
+                    DashboardSummary = previous.DashboardSummary with
+                    {
+                        Status = DashboardGalaxyStatus.Healthy,
+                        LastQueriedAt = queriedAt,
+                        LastSuccessAt = queriedAt,
+                        LastDeployTime = deployTime,
+                        LastError = null,
+                    },
                 };
                 Volatile.Write(ref _current, refreshed);
                 _firstLoad.TrySetResult();

src/MxGateway.Server/Grpc/GalaxyRepositoryGrpcService.cs

@@ -68,7 +68,8 @@ public sealed class GalaxyRepositoryGrpcService(
                 ResolveUnavailableMessage(entry)));
         }
 
-        int offset = ParsePageToken(request.PageToken);
+        PageToken pageToken = ParsePageToken(request.PageToken, entry.Sequence);
+        int offset = pageToken.Offset;
         if (offset > entry.Objects.Count)
         {
             throw new RpcException(new Status(
@@ -90,7 +91,7 @@ public sealed class GalaxyRepositoryGrpcService(
         int nextOffset = offset + take;
         if (nextOffset < entry.Objects.Count)
         {
-            reply.NextPageToken = nextOffset.ToString(System.Globalization.CultureInfo.InvariantCulture);
+            reply.NextPageToken = FormatPageToken(entry.Sequence, nextOffset);
         }
 
         return reply;
@@ -182,15 +183,30 @@ public sealed class GalaxyRepositoryGrpcService(
         return Math.Min(pageSize, MaxDiscoverPageSize);
     }
 
-    private static int ParsePageToken(string pageToken)
+    private static string FormatPageToken(long sequence, int offset)
+    {
+        return string.Concat(
+            sequence.ToString(System.Globalization.CultureInfo.InvariantCulture),
+            ":",
+            offset.ToString(System.Globalization.CultureInfo.InvariantCulture));
+    }
+
+    private static PageToken ParsePageToken(string pageToken, long currentSequence)
     {
         if (string.IsNullOrWhiteSpace(pageToken))
         {
-            return 0;
+            return new PageToken(currentSequence, Offset: 0);
         }
 
-        if (!int.TryParse(
-                pageToken,
+        string[] parts = pageToken.Split(':', count: 2);
+        if (parts.Length != 2
+            || !long.TryParse(
+                parts[0],
+                System.Globalization.NumberStyles.None,
+                System.Globalization.CultureInfo.InvariantCulture,
+                out long sequence)
+            || !int.TryParse(
+                parts[1],
                 System.Globalization.NumberStyles.None,
                 System.Globalization.CultureInfo.InvariantCulture,
                 out int offset)
@@ -201,9 +217,18 @@ public sealed class GalaxyRepositoryGrpcService(
                 "DiscoverHierarchy page_token is invalid."));
         }
 
-        return offset;
+        if (sequence != currentSequence)
+        {
+            throw new RpcException(new Status(
+                StatusCode.InvalidArgument,
+                "DiscoverHierarchy page_token is stale."));
+        }
+
+        return new PageToken(sequence, offset);
     }
 
+    private sealed record PageToken(long Sequence, int Offset);
+
     [System.Diagnostics.CodeAnalysis.SuppressMessage(
         "Style",
         "IDE0051:Remove unused private members",