chore: Fix langchain_core test that used a new Sequence type

[schloerke]

langchain_core was updated to 0.3.2 on Sep 19th, and errors started popping up on Sept 20th

Error being seen in CI:

=================================== FAILURES ===================================
__________________________ test_as_langchain_message ___________________________
[gw0] linux -- Python 3.12.6 /opt/hostedtoolcache/Python/3.12.6/x64/bin/python3

    def test_as_langchain_message():
        from langchain_core.language_models.base import LanguageModelInput
        from langchain_core.language_models.chat_models import BaseChatModel
        from langchain_core.messages import (
            AIMessage,
            BaseMessage,
            HumanMessage,
            MessageLikeRepresentation,
            SystemMessage,
        )
    
        from shiny.ui._chat_provider_types import as_langchain_message
    
        assert BaseChatModel.invoke.__annotations__["input"] == "LanguageModelInput"
        assert BaseChatModel.stream.__annotations__["input"] == "LanguageModelInput"
    
>       assert is_type_in_union(Sequence[MessageLikeRepresentation], LanguageModelInput)
E       assert False
E        +  where False = is_type_in_union(typing.Sequence[typing.Union[langchain_core.messages.base.BaseMessage, list[str], tuple[str, str], str, dict[str, typing.Any]]], typing.Union[langchain_core.prompt_values.PromptValue, str, collections.abc.Sequence[typing.Union[langchain_core.messages.base.BaseMessage, list[str], tuple[str, str], str, dict[str, typing.Any]]]])

[schloerke]

Found root cause: langchain-ai/langchain@a47b332#diff-639e6297e1db3729c228f5172ad900f6b5385c04ffef072fba2c4acd76de7e8dR4

[schloerke]

Reviewed in person with @karangattu