fix: place collapsed nodes inside schema clusters for proper layout

Collapsed nodes now include schema_name attribute and are added to
schema_map so they appear inside the cluster with other tables from
the same schema. This fixes the visual layout so collapsed middle
layers appear between top and bottom tables, maintaining DAG flow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -514,14 +514,19 @@ def _apply_collapse(self, graph: nx.DiGraph) -> tuple[nx.DiGraph, dict[str, str]
                                 continue
                     node_mapping[node] = node
 
+            # Build reverse mapping: label -> schema_name
+            label_to_schema = {label: schema for schema, label in collapsed_labels.items()}
+
             # Add nodes
             added_collapsed = set()
             for old_node, new_node in node_mapping.items():
                 if new_node in collapsed_counts:
                     # This is a collapsed schema node
                     if new_node not in added_collapsed:
+                        schema_name = label_to_schema.get(new_node, new_node)
                         new_graph.add_node(new_node, node_type=None, collapsed=True,
-                                          table_count=collapsed_counts[new_node])
+                                          table_count=collapsed_counts[new_node],
+                                          schema_name=schema_name)
                         added_collapsed.add(new_node)
                 else:
                     new_graph.add_node(new_node, **graph.nodes[old_node])
@@ -660,6 +665,11 @@ def make_dot(self):
                     if successors and successors[0] in schema_map:
                         schema_map[node] = schema_map[successors[0]]
 
+            # Assign collapsed nodes to their schema so they appear in the cluster
+            for node, data in graph.nodes(data=True):
+                if data.get("collapsed") and data.get("schema_name"):
+                    schema_map[node] = data["schema_name"]
+
             scale = 1.2  # scaling factor for fonts and boxes
             label_props = {  # http://matplotlib.org/examples/color/named_colors.html
                 None: dict(