Updating the unit tests to catch up with langchain-core. No adjustements
should be necessary to user code, the issues manifest themselves only with the
given test set up (e.g., snapshots). langchain-core changes were either
additive or self-consistent.

This PR upgrades langserve to pydantic 2.

* Added a failing unit test that has 2 known failures (that need to be
fixed in langchain-core)
* Deprecation warnings will be resolved separately.

Migrate examples to pydantic 2

…de) (#746)

Update the chat playground to handle the oneOf type which appears as the
input into language models.

This change stems from the upgrade to pydantic 2.

Update the default playground to handle the oneOf type which appears as
the input into language models.

This change stems from the upgrade to pydantic 2.