fix: don't collapse fresh diagrams that were never combined

A fresh dj.Diagram(schema) was incorrectly collapsing because
_explicit_nodes was empty. Now we check both _explicit_nodes and
_is_collapsed to determine if collapse should be applied:
- Fresh diagram (_explicit_nodes empty, _is_collapsed=False): no collapse
- Combined collapsed (_explicit_nodes empty, _is_collapsed=True): collapse all
- Mixed combination: collapse only non-explicit nodes

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

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -386,6 +386,15 @@ def _apply_collapse(self, graph: nx.DiGraph) -> tuple[nx.DiGraph, dict[str, str]
             valid_nodes = self.nodes_to_show.intersection(set(self.nodes()))
             valid_explicit = self._explicit_nodes.intersection(set(self.nodes()))
 
+            # Determine if collapse should be applied:
+            # - If _explicit_nodes is empty AND _is_collapsed is False, this is a fresh
+            #   diagram that was never combined with collapsed diagrams → no collapse
+            # - If _explicit_nodes is empty AND _is_collapsed is True, this is the result
+            #   of combining only collapsed diagrams → collapse all nodes
+            # - If _explicit_nodes equals valid_nodes, all nodes are explicit → no collapse
+            if not valid_explicit and not self._is_collapsed:
+                # Fresh diagram, never combined with collapsed diagrams
+                return graph, {}
             if valid_explicit == valid_nodes:
                 # All nodes are explicit (expanded) - no collapse needed
                 return graph, {}