grpc: make page_token error strings RPC-name-agnostic

src/ZB.MOM.WW.MxGateway.Server/Grpc/GalaxyRepositoryGrpcService.cs

@@ -348,21 +348,21 @@ public sealed class GalaxyRepositoryGrpcService(
         {
             throw new RpcException(new Status(
                 StatusCode.InvalidArgument,
-                "DiscoverHierarchy page_token is invalid."));
+                "page_token is invalid."));
         }
 
         if (sequence != currentSequence)
         {
             throw new RpcException(new Status(
                 StatusCode.InvalidArgument,
-                "DiscoverHierarchy page_token is stale."));
+                "page_token is stale."));
         }
 
         if (!string.Equals(parts[1], currentFilterSignature, StringComparison.Ordinal))
         {
             throw new RpcException(new Status(
                 StatusCode.InvalidArgument,
-                "DiscoverHierarchy page_token does not match the current filters."));
+                "page_token does not match the current filters."));
         }
 
         return new PageToken(sequence, parts[1], offset);