core[patch]: fix init of RunnableAssign (#26903)

Example in API ref currently raises ValidationError.

Resolves #26862

Author: ccurme

libs/core/langchain_core/runnables/passthrough.py

@@ -389,7 +389,7 @@ def add_ten(x: Dict[str, int]) -> Dict[str, int]:
             # returns {'input': 5, 'add_step': {'added': 15}}
     """
 
-    mapper: RunnableParallel[dict[str, Any]]
+    mapper: RunnableParallel
 
     def __init__(self, mapper: RunnableParallel[dict[str, Any]], **kwargs: Any) -> None:
         super().__init__(mapper=mapper, **kwargs)  # type: ignore[call-arg]

libs/core/tests/unit_tests/runnables/test_runnable.py

@@ -63,6 +63,7 @@
     ConfigurableFieldSingleOption,
     RouterRunnable,
     Runnable,
+    RunnableAssign,
     RunnableBinding,
     RunnableBranch,
     RunnableConfig,
@@ -5413,3 +5414,14 @@ def test_schema_for_prompt_and_chat_model() -> None:
         "title": "PromptInput",
         "type": "object",
     }
+
+
+def test_runnable_assign() -> None:
+    def add_ten(x: dict[str, int]) -> dict[str, int]:
+        return {"added": x["input"] + 10}
+
+    mapper = RunnableParallel({"add_step": RunnableLambda(add_ten)})
+    runnable_assign = RunnableAssign(mapper)
+
+    result = runnable_assign.invoke({"input": 5})
+    assert result == {"input": 5, "add_step": {"added": 15}}