feat(prebuilt): support ToolOutput response_format (#5915)

* Add support for ToolOutput response format.
* I don't love the name -- it's confusing unless you know that it's parameterizing a strategy.

We should determine if we want to support our old strategy for doing
things -- it has a higher latency (one extra LLM call), but it's a
reasonable built-in strategy as it doesn't do anything awkward with
conversation history. (Wouldn't surprising if it has overall better
performance than tool choice for longer conversations)

libs/prebuilt/tests/test_react_agent_graph.py

@@ -39,20 +39,17 @@ class ResponseFormat(BaseModel):
 @pytest.mark.parametrize("tools", [[], [tool]])
 @pytest.mark.parametrize("pre_model_hook", [None, pre_model_hook])
 @pytest.mark.parametrize("post_model_hook", [None, post_model_hook])
-@pytest.mark.parametrize("response_format", [None, ResponseFormat])
 def test_react_agent_graph_structure(
     snapshot: SnapshotAssertion,
     tools: list[Callable],
     pre_model_hook: Union[Callable, None],
     post_model_hook: Union[Callable, None],
-    response_format: Union[type[BaseModel], None],
 ) -> None:
     agent = create_react_agent(
         model,
         tools=tools,
         pre_model_hook=pre_model_hook,
         post_model_hook=post_model_hook,
-        response_format=response_format,
     )
     try:
         assert agent.get_graph().draw_mermaid(with_styles=False) == snapshot
@@ -63,7 +60,6 @@ def test_react_agent_graph_structure(
             f"tools: {tools}, "
             f"pre_model_hook: {pre_model_hook}, "
             f"post_model_hook: {post_model_hook}, "
-            f"response_format: {response_format}"
         ) from e
 
 
@@ -74,24 +70,17 @@ def test_react_agent_graph_structure(
 @pytest.mark.parametrize(
     "post_model_hook", [None, post_model_hook], ids=["no_post_hook", "with_post_hook"]
 )
-@pytest.mark.parametrize(
-    "response_format",
-    [None, ResponseFormat],
-    ids=["no_response_format", "with_response_format"],
-)
 def test_react_agent_graph_structure_with_individual_nodes(
     snapshot: SnapshotAssertion,
     tools: list[Callable],
     pre_model_hook: Union[Callable, None],
     post_model_hook: Union[Callable, None],
-    response_format: Union[type[BaseModel], None],
 ) -> None:
     agent = create_react_agent(
         model,
         tools=tools,
         pre_model_hook=pre_model_hook,
         post_model_hook=post_model_hook,
-        response_format=response_format,
         use_individual_tool_nodes=True,
     )
     assert agent.get_graph().draw_mermaid(with_styles=False) == snapshot