fix(#1423): gate Diagram.__getitem__ on _mode == "trace"

The trace-mode __getitem__ I added shadowed networkx.DiGraph's standard
adjacency-dict lookup for ALL Diagrams, not just trace results. ERD
tests (and any other code that does diagram[node_name] for adjacency)
were getting DataJointError("not in this trace's subgraph") instead of
the adjacency dict.

Fix: short-circuit non-trace diagrams (no _mode attribute or _mode != "trace")
to super().__getitem__(key) before any trace-specific logic runs.

Tests:
- 5 previously-failing erd tests now pass (test_erd, test_diagram_algebra,
  test_repr_svg, test_make_image, test_part_table_parsing).
- 8/8 trace tests still pass.

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -551,6 +551,10 @@ def __getitem__(self, key):
         Return a pre-restricted query expression (or FreeTable) for an
         ancestor table in this trace.
 
+        Only meaningful for trace diagrams (constructed via
+        :meth:`Diagram.trace`). For ordinary diagrams, defers to
+        :class:`networkx.DiGraph`'s adjacency-dict lookup.
+
         Parameters
         ----------
         key : type or str
@@ -577,6 +581,12 @@ def __getitem__(self, key):
         >>> trace[Session].fetch1("session_date")     # class index
         >>> trace["my_schema.Session"].to_dicts()     # string index → FreeTable
         """
+        # Non-trace diagrams: defer to networkx adjacency lookup so existing
+        # `diagram[node_name]` patterns (used in diagram algebra, ERD tests)
+        # keep working.
+        if getattr(self, "_mode", None) != "trace":
+            return super().__getitem__(key)
+
         from .table import Table
 
         # Resolve `key` to a full table name