fix: ensure default edge is not removed

If a new edge update comes in that sets a schema variant to the default,
but that variant is already default, we end up with no default.

I'm not sure why this update is being produced (since there should be no
difference in this case for the edge in question), but this prevents the
issue from arising.

lib/dal/src/workspace_snapshot/graph/v4.rs

@@ -1631,7 +1631,7 @@ impl WorkspaceSnapshotGraphV4 {
                     if let (Some(source_idx), Some(destination_idx)) = (source_idx, destination_idx)
                     {
                         if let EdgeWeightKind::Use { is_default: true } = edge_weight.kind() {
-                            ensure_only_one_default_use_edge(self, source_idx)?;
+                            ensure_only_one_default_use_edge(self, source_idx, destination_idx)?;
                         }
 
                         self.add_edge_inner(
@@ -1781,14 +1781,15 @@ fn prop_node_indexes_for_node_index(
 fn ensure_only_one_default_use_edge(
     graph: &mut WorkspaceSnapshotGraphV4,
     source_idx: NodeIndex,
+    destination_idx: NodeIndex,
 ) -> WorkspaceSnapshotGraphResult<()> {
     let existing_default_targets: Vec<NodeIndex> = graph
         .edges_directed(source_idx, Outgoing)
         .filter(|edge_ref| {
             matches!(
                 edge_ref.weight().kind(),
                 EdgeWeightKind::Use { is_default: true }
-            )
+            ) && edge_ref.target() != destination_idx
         })
         .map(|edge_ref| edge_ref.target())
         .collect();