feat(prebuilt): Split tool node to individual tool nodes (#5888)

Add option to split tool node to individual nodes. 

Summary:
* User code (specifically streaming) may break if it's relying on the
name of the `tools` node
* The boolean flag in the interface is likely **temporary** (especially
if there are no major breaking changes)
* We'll need to decide if we can get rid of the version in create react
agent. "v1" is not consistent conceptually with a node per tool.

libs/prebuilt/tests/test_react_agent.py

@@ -780,14 +780,24 @@ class AgentStateExtraKeyPydantic(AgentStatePydantic):
     foo: int
 
 
-@pytest.mark.parametrize("version", REACT_TOOL_CALL_VERSIONS)
+@pytest.mark.parametrize("version", ["v1", "v2"])
 @pytest.mark.parametrize(
     "state_schema", [AgentStateExtraKey, AgentStateExtraKeyPydantic]
 )
+@pytest.mark.parametrize(
+    "use_individual_tool_nodes",
+    [False, True],
+    ids=["single_tool_node", "node_per_tool"],
+)
 def test_create_react_agent_inject_vars(
-    version: Literal["v1", "v2"], state_schema: StateSchemaType
+    version: Literal["v1", "v2"],
+    state_schema: StateSchemaType,
+    use_individual_tool_nodes: bool,
 ) -> None:
     """Test that the agent can inject state and store into tool functions."""
+    if version == "v1" and use_individual_tool_nodes:
+        pytest.skip("v1 does not support individual tool nodes")
+
     store = InMemoryStore()
     namespace = ("test",)
     store.put(namespace, "test_key", {"bar": 3})
@@ -826,6 +836,7 @@ def test_create_react_agent_inject_vars(
         state_schema=state_schema,
         store=store,
         version=version,
+        use_individual_tool_nodes=use_individual_tool_nodes,
     )
     result = agent.invoke({"messages": [{"role": "user", "content": "hi"}], "foo": 2})
     assert result["messages"] == [
@@ -967,7 +978,17 @@ def test_tool_node_messages_key() -> None:
 
 
 @pytest.mark.parametrize("version", REACT_TOOL_CALL_VERSIONS)
-async def test_return_direct(version: str) -> None:
+@pytest.mark.parametrize(
+    "use_individual_tool_nodes",
+    [False, True],
+    ids=["single_tool_node", "node_per_tool"],
+)
+async def test_return_direct(
+    version: Literal["v1", "v2"], use_individual_tool_nodes: bool
+) -> None:
+    if version == "v1" and use_individual_tool_nodes:
+        pytest.skip("v1 does not support individual tool nodes")
+
     @dec_tool(return_direct=True)
     def tool_return_direct(input: str) -> str:
         """A tool that returns directly."""
@@ -995,6 +1016,7 @@ async def test_return_direct(version: str) -> None:
         model,
         [tool_return_direct, tool_normal],
         version=version,
+        use_individual_tool_nodes=use_individual_tool_nodes,
     )
 
     # Test direct return for tool_return_direct
@@ -1088,15 +1110,27 @@ def test__get_state_args() -> None:
 
 
 def test_inspect_react() -> None:
+    """Test that we can inspect the agent and its nodes."""
     model = FakeToolCallingModel(tool_calls=[])
     agent = create_react_agent(model, [])
     inspect.getclosurevars(agent.nodes["agent"].bound.func)
 
 
 @pytest.mark.parametrize("version", REACT_TOOL_CALL_VERSIONS)
+@pytest.mark.parametrize(
+    "use_individual_tool_nodes",
+    [False, True],
+    ids=["single_tool_node", "node_per_tool"],
+)
 def test_react_with_subgraph_tools(
-    sync_checkpointer: BaseCheckpointSaver, version: Literal["v1", "v2"]
+    sync_checkpointer: BaseCheckpointSaver,
+    version: Literal["v1", "v2"],
+    use_individual_tool_nodes: bool,
 ) -> None:
+    """Test React agent with subgraph tools."""
+    if version == "v1" and use_individual_tool_nodes:
+        pytest.skip("v1 does not support individual tool nodes")
+
     class State(TypedDict):
         a: int
         b: int
@@ -1152,6 +1186,7 @@ def test_react_with_subgraph_tools(
         tool_node,
         checkpointer=sync_checkpointer,
         version=version,
+        use_individual_tool_nodes=use_individual_tool_nodes,
     )
     result = agent.invoke(
         {"messages": [HumanMessage(content="What's 2 + 3 and 2 * 3?")]},
@@ -1237,8 +1272,17 @@ def test_tool_node_stream_writer() -> None:
 
 
 @pytest.mark.parametrize("version", REACT_TOOL_CALL_VERSIONS)
-def test_react_agent_subgraph_streaming_sync(version: Literal["v1", "v2"]) -> None:
+@pytest.mark.parametrize(
+    "use_individual_tool_nodes",
+    [False, True],
+    ids=["single_tool_node", "node_per_tool"],
+)
+def test_react_agent_subgraph_streaming_sync(
+    version: Literal["v1", "v2"], use_individual_tool_nodes: bool
+) -> None:
     """Test React agent streaming when used as a subgraph node sync version"""
+    if version == "v1" and use_individual_tool_nodes:
+        pytest.skip("v1 does not support individual tool nodes")
 
     @dec_tool
     def get_weather(city: str) -> str:
@@ -1258,6 +1302,7 @@ def test_react_agent_subgraph_streaming_sync(version: Literal["v1", "v2"]) -> No
         tools=[get_weather],
         prompt="You are a helpful travel assistant.",
         version=version,
+        use_individual_tool_nodes=use_individual_tool_nodes,
     )
 
     # Create a subgraph that uses the React agent as a node