fix: Update LangChain imports for v1.x compatibility (#2700)

Fixes #2607

- Replace deprecated `langchain.*` imports with `langchain_core.*`
- Updates imports for Run, dumps, and BaseMessage
- Resolves ModuleNotFoundError when using LangchainCallbackHandler with
LangChain v1.x

**Note:** This PR uses the correct `langchain_core.tracers.schemas.Run`
import, which is part of LangChain's core tracer interface. Another PR
(#2695) uses `langsmith.schemas.Run`, but `langchain_core` is the proper
source as it contains the base abstractions that LangChain's callback
system is built on, while `langsmith` is specifically for the LangSmith
monitoring service integration.




<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Updated LangChain imports to use langchain_core so the
LangchainCallbackHandler works with LangChain v1.x. Fixes #2607 by
resolving ModuleNotFoundError from deprecated langchain.* imports
(dumps, BaseMessage, Run).

<sup>Written for commit 7fed064.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

Co-authored-by: Josh Hayes <[email protected]>

Author: RoyBA

backend/chainlit/langchain/callbacks.py

@@ -3,11 +3,11 @@
 from uuid import UUID
 
 import pydantic
-from langchain.callbacks.tracers.schemas import Run
-from langchain.load.dump import dumps
-from langchain.schema import BaseMessage
+from langchain_core.load import dumps
+from langchain_core.messages import BaseMessage
 from langchain_core.outputs import ChatGenerationChunk, GenerationChunk
 from langchain_core.tracers.base import AsyncBaseTracer
+from langchain_core.tracers.schemas import Run
 from literalai import ChatGeneration, CompletionGeneration, GenerationMessage
 from literalai.observability.step import TrueStepType