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

Add missing imports in docs from langchain_core.tools  BaseTool

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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)