docs: Update how to docs for pydantic compatibility (#22983)

Add missing imports in docs from langchain_core.tools  BaseTool

---------

Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>

docs/docs/how_to/pydantic_compatibility.md

@@ -22,6 +22,7 @@ the case of inheritance and in the case of passing objects to LangChain.
 
 ```python
 from pydantic.v1 import root_validator, validator
+from langchain_core.tools import BaseTool
 
 class CustomTool(BaseTool): # BaseTool is v1 code
     x: int = Field(default=1)
@@ -48,6 +49,7 @@ Mixing Pydantic v2 primitives with Pydantic v1 primitives can raise cryptic erro
 
 ```python
 from pydantic import Field, field_validator # pydantic v2
+from langchain_core.tools import BaseTool
 
 class CustomTool(BaseTool): # BaseTool is v1 code
     x: int = Field(default=1)
@@ -102,4 +104,4 @@ Tool.from_function( # <-- tool uses v1 namespace
     description="useful for when you need to answer questions about math",
     args_schema=CalculatorInput
 )
-```
+```