core[patch]: Add default None to StructuredTool func (#26324)

This PR was autogenerated using gritql, tests written manually

```shell

grit apply 'class_definition(name=$C, $body, superclasses=$S) where {    
    $C <: ! "Config", // Does not work in this scope, but works after class_definition
    $body <: block($statements),
    $statements <: some bubble assignment(left=$x, right=$y, type=$t) as $A where {
        or {
            $y <: `Field($z)`,
            $x <: "model_config"
        }
    },
    // And has either Any or Optional fields without a default
    $statements <: some bubble assignment(left=$x, right=$y, type=$t) as $A where {
        $t <: or {
            r"Optional.*",
            r"Any",
            r"Union[None, .*]",
            r"Union[.*, None, .*]",
            r"Union[.*, None]",
        },
        $y <: ., // Match empty node        
        $t => `$t = None`,
    },    
}
' --language python .

```

libs/core/langchain_core/tools/structured.py

@@ -40,7 +40,7 @@ class StructuredTool(BaseTool):
         ..., description="The tool schema."
     )
     """The input arguments' schema."""
-    func: Optional[Callable[..., Any]]
+    func: Optional[Callable[..., Any]] = None
     """The function to run when the tool is called."""
     coroutine: Optional[Callable[..., Awaitable[Any]]] = None
     """The asynchronous version of the function."""
@@ -98,8 +98,8 @@ class StructuredTool(BaseTool):
                 kwargs[config_param] = config
             return await self.coroutine(*args, **kwargs)
 
-        # NOTE: this code is unreachable since _arun is only called if coroutine is not
-        # None.
+        # If self.coroutine is None, then this will delegate to the default
+        # implementation which is expected to delegate to _run on a separate thread.
         return await super()._arun(
             *args, config=config, run_manager=run_manager, **kwargs
         )

libs/core/tests/unit_tests/test_tools.py

@@ -54,7 +54,6 @@ from langchain_core.tools.base import (
 )
 from langchain_core.utils.function_calling import convert_to_openai_function
 from langchain_core.utils.pydantic import PYDANTIC_MAJOR_VERSION, _create_subset_model
-from langchain_core.utils.pydantic import TypeBaseModel as TypeBaseModel
 from tests.unit_tests.fake.callbacks import FakeCallbackHandler
 from tests.unit_tests.pydantic_utils import _schema
 
@@ -1978,3 +1977,19 @@ def test_imports() -> None:
     ]
     for module_name in expected_all:
         assert hasattr(tools, module_name) and getattr(tools, module_name) is not None
+
+
+def test_structured_tool_direct_init() -> None:
+    def foo(bar: str) -> str:
+        return bar
+
+    async def asyncFoo(bar: str) -> str:
+        return bar
+
+    class fooSchema(BaseModel):
+        bar: str = Field(..., description="The bar")
+
+    tool = StructuredTool(name="foo", args_schema=fooSchema, coroutine=asyncFoo)
+
+    with pytest.raises(NotImplementedError):
+        assert tool.invoke("hello") == "hello"