test(langgraph): mark node-exit state suppression as a producer choice

The assertion message said "a subagent must not emit STATE_SNAPSHOT; only the
parent owns state", stating the removed rule as a conformance requirement. The
suppression itself is correct and stays -- a subgraph's state is a partial view
of the run's document, so emitting one mid-delegation would overwrite the whole
state with a fragment -- but that is a choice about this integration's state
model, not something the protocol demands.

Also drops the second assertion, which required that NO emitted STATE_SNAPSHOT
carry a subagent id. That is no longer a valid invariant: the explicit
manually_emit_state path deliberately emits an attributed snapshot now. It passed
only because this test drives the node-exit path, so it was pinning an accident
rather than a property.

Verified: 468 LangGraph tests.

Author: mme

integrations/langgraph/python/tests/test_subagent_emission.py

@@ -478,17 +478,18 @@ def _node_exit_snapshots(collected):
         ]
 
     async def test_node_exit_state_snapshot_suppressed_inside_subagent(self):
+        # This pins a PRODUCER-SIDE choice, not a protocol rule. Attributed state
+        # events are legal (the design lists STATE_* as attributable, and the
+        # explicit manually_emit_state path does emit them). Node-exit snapshots are
+        # suppressed because a subgraph's state is a PARTIAL view of the run's
+        # document, so emitting one mid-delegation would overwrite the whole state
+        # with a fragment. If the integration's state model ever changes, this test
+        # should change with it rather than being treated as a conformance rule.
         collected = await self._drive(in_subagent=True)
         self.assertEqual(
             self._node_exit_snapshots(collected), [],
-            "a subagent must not emit STATE_SNAPSHOT; only the parent owns state",
-        )
-        # And nothing that did go out claims to be the subagent's state.
-        self.assertEqual(
-            [e for e in collected
-             if getattr(e, "type", None) == EventType.STATE_SNAPSHOT
-             and getattr(e, "subagent_run_id", None) is not None],
-            [],
+            "node-exit snapshots carry a partial subgraph view, so they are "
+            "suppressed mid-delegation",
         )
 
     async def test_node_exit_state_snapshot_emitted_for_parent(self):