test: add explicit coverage for _InvokeSession async send_to_parent

Ensures the `isawaitable` branch in `_InvokeSession.send_to_parent`
is covered when the parent machine uses an async engine, fixing a
coverage drop detected by Codecov.

Author: fgmacedo

tests/test_scxml_units.py

@@ -1031,6 +1031,21 @@ def run(self, ctx):
         assert wrapper._instance is handler_instance
 
 
+class TestInvokeSessionSendToParentAsync:
+    async def test_send_to_parent_awaitable(self):
+        """send_to_parent calls ensure_future when parent.send returns an awaitable."""
+        from statemachine.io.scxml.invoke import _InvokeSession
+
+        async def async_send(event, **kwargs):
+            pass
+
+        parent = Mock()
+        parent.send = async_send
+        session = _InvokeSession(parent, invokeid="inv1")
+        # Should not raise — the coroutine is scheduled via ensure_future
+        session.send_to_parent("child.done", key="value")
+
+
 class TestOrderedSetStr:
     def test_str_representation(self):
         """OrderedSet.__str__ returns a set-like string."""