fix(core): enforce langchain <0.3 for pydantic v1 (#3217)

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

backend/api/pyproject.toml

@@ -33,7 +33,7 @@ dependencies = [
     "markdownify>=0.13.1",
     "langchain-openai>=0.1.21",
     "resend>=2.4.0",
-    "langchain>=0.2.14",
+    "langchain>=0.2.14,<0.3.0",
     "litellm>=1.43.15",
     "openai>=1.40.8",
     "tiktoken>=0.7.0",

backend/core/pyproject.toml

@@ -8,7 +8,7 @@ authors = [
 dependencies = [
     "pydantic>=2.8.2",
     "langchain-core>=0.2.38",
-    "langchain>=0.2.14",
+    "langchain>=0.2.14,<0.3.0",
     "langgraph>=0.2.14",
     "httpx>=0.27.0",
     "rich>=13.7.1",