Upgraded to langchain 0.3.x

[patcheng]

Description: Langchain 0.3.x upgraded pydantic from v1 to v2, so need to migrate this project's to use Pydantic v2.

I upgraded langchain packages to 0.3.x. They no longer supports Python 3.8, so upgraded that as well. Followed commit message https://github.com/langchain-ai/langchain/pull/26443 and followed the steps in https://github.com/eyurtsev/migrate-pydantic/tree/main?tab=readme-ov-file.

Fixed up various model so the tests passed.

Some noteable changes:

• The order of execution for validator changes between v1 and v2.
• also, seems like validator with the same name is base class is not being called anymore, so need to rename the method.

Ran make test and everything pass. Ran make lint and fixed errors related to the change, and some obvious errors. There are still some errors that might be existing? Ran make format

I tried to do each step as a separate commit. Hopefully makes it slightly easier to review.

[pprados]

Thanks!

To validate this PR, the make validate command must not return any errors. Can you fix the last points?

[patcheng]

ok. test, lint, format and validate are all passing.

[pprados]

Very good job !

[pprados]

The version v0.3.0 is published!