Fix UnionType type var replacement (#25566)

[langchain_core] Fix UnionType type var replacement

- Added types.UnionType to typing.Union mapping

Type replacement cause `TypeError: 'type' object is not subscriptable`
if any of union type comes as function `_py_38_safe_origin` return
`types.UnionType` instead of `typing.Union`

```python
>>> from types import UnionType
>>> from typing import Union, get_origin
>>> type_ = get_origin(str | None)
>>> type_
<class 'types.UnionType'>
>>> UnionType[(str, None)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'type' object is not subscriptable
>>> Union[(str, None)]
typing.Optional[str]
```

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/core/tests/unit_tests/utils/test_function_calling.py

@@ -1,4 +1,5 @@
 # mypy: disable-error-code="annotation-unchecked"
+import sys
 from typing import (
     Any,
     Callable,
@@ -702,3 +703,18 @@ def test__convert_typed_dict_to_openai_function_fail(typed_dict: Type) -> None:
 
     with pytest.raises(TypeError):
         _convert_typed_dict_to_openai_function(Tool)
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 10), reason="Requires python version >= 3.10 to run."
+)
+def test_convert_union_type_py_39() -> None:
+    @tool
+    def magic_function(input: int | float) -> str:
+        """Compute a magic function."""
+        pass
+
+    result = convert_to_openai_function(magic_function)
+    assert result["parameters"]["properties"]["input"] == {
+        "anyOf": [{"type": "integer"}, {"type": "number"}]
+    }