bug[core]: fix tool descriptions when BaseModels have no docstring

[ccurme]

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Issue Content

When Pydantic models are cast to BaseTool and are missing a docstring, we use the docstring for BaseModel itself:

from langchain_core.tools import tool
from pydantic import BaseModel


@tool
class MyTool(BaseModel):
    foo: str


MyTool.description

'!!! abstract "Usage Documentation"\n Models\n\nA base class for creating Pydantic models.\n\nAttributes:\n class_vars: The names of the class variables defined on the model.\n private_attributes: Metadata about...'

We should use an empty string in this case.

[keenborder786]

@ccurme patched

[eyurtsev]

from langchain_core.tools import tool
from pydantic import BaseModel


class MyTool(BaseModel):
    """Parent Tool"""
    foo: str

@tool
class ChildTool(MyTool):
    bar: str

For 0.4 release we could consider updating the behavior so that doc-strings are not inherited from their parents. It's fairly odd that doc-strings are inherited for BaseModels as in general the descriptions need to be different