Support LangChain 0.3.0

[B-Step62]

Update pydantic related logic to be compatible with LangChain 0.3.0.

Testing
Validated on notebook with different env settings:

langchain                          0.3.0
langchain-core                     0.3.1
pydantic                           2.8.2
pydantic_core                      2.20.1

langchain                          0.2.16
langchain-core                     0.2.40
pydantic                           2.8.2
pydantic_core                      2.20.1

langchain                          0.2.16
langchain-core                     0.2.40
pydantic                           1.10.18
pydantic_core                      0.42.0

[B-Step62]

@efriis Let me tag you for double-checking if there is anything missing for supporting LangChain 0.3.0. Thank you!

[B-Step62]

note: After updating to pydantic v2 import, this field starts to return FieldInfo object as default value instead of factory. Might be pydantic issue, but I couldn't figure it out the root cause so just using None as a workaround.

[B-Step62]

note: The langchain-core bumps minimum Python version. We need to follow it otherwise poetry complains.

[harupy]

LGTM!