fix(structured): harden structured output for local servers and thinking models

- Local servers (LM Studio, vLLM) reject the object-form tool_choice langchain
  sends for function calling. The generic openai_compatible provider now binds
  the schema as a tool without forcing tool_choice.
- A structured call can return no parsed result (a thinking model answering in
  plain text); fall back to free text with a clear reason instead of an opaque
  render error.

tests/test_structured_agents.py

@@ -121,6 +121,24 @@ def _structured_trader_llm(captured: dict, proposal: TraderProposal | None = Non
     return llm
 
 
+@pytest.mark.unit
+def test_invoke_structured_falls_back_when_result_is_none():
+    # A thinking model can answer in plain text, leaving the parser with None.
+    # That must fall back to free text, not crash on render(None) (#1051).
+    from tradingagents.agents.utils.structured import invoke_structured_or_freetext
+
+    structured = MagicMock()
+    structured.invoke.return_value = None
+    plain = MagicMock()
+    plain.invoke.return_value = MagicMock(content="FREETEXT")
+
+    out = invoke_structured_or_freetext(
+        structured, plain, "prompt", render=lambda r: r.rating, agent_name="t"
+    )
+    assert out == "FREETEXT"
+    plain.invoke.assert_called_once()
+
+
 @pytest.mark.unit
 class TestTraderAgent:
     def test_structured_path_produces_rendered_markdown(self):