Use keyed parameters in SubagentTextResult constructor

I have not rebased #755, but after merge it conflicted with #758.

Author: mateusz834

tests/integration/ai/test_agent_message_validation.py

@@ -509,7 +509,7 @@ class _AlienStructuredOutputCall(StructuredOutputCall):
                     SubagentMessage(
                         name="my_agent",
                         call_id="id-1",
-                        result=SubagentTextResult("foo"),
+                        result=SubagentTextResult(content="foo"),
                     ),
                 ],
                 "thread_id should not be an empty string",