fix: upgrade langchain-core to address CVE-2025-68664

[afida23]

What changed

Bumps langchain-core 0.2.1 → 0.3.81 and pydantic 2.4.0 → 2.7.4 in both backend requirements files of the gen-ai-chatbot blueprint. langchain-core versions before 0.3.81 are vulnerable to CVE-2025-68664; pydantic must be >=2.7.4 for langchain-core 0.3.x.

langchain-core 0.3 moves its base classes to native pydantic v2, which silently disables v1-style validators registered via the langchain_core.pydantic_v1 shim. Five files under app/agents/ are migrated to pydantic v2 APIs (@model_validator/@field_validator, ConfigDict, model_fields) so the agent feature keeps working.

Testing

• Dependency resolution verified for both requirements files (Python 3.11)
• Ran the backend unit suite on the new pins and compared against a baseline run on the old pins: same pass/fail results, with the remaining failures pre-existing (tests needing deployed AWS infra). Without the validator migration, AgentExecutor construction fails on langchain-core 0.3.x (test_create_react_agent); with it, the test passes construction again
• Ran the blueprint's Bedrock integration tests (test_langchain.py, test_stream.py) with live AWS credentials: model invocation and streaming succeed through the migrated agent code on langchain-core 0.3.81 (Mistral variants; the Claude variants fail only because the tests hardcode the now-retired claude-v3-haiku model, unrelated to this change)
• requirements.txt installs cleanly on the python:3.11.6-slim-bookworm base image

Issues

• https://t.corp.amazon.com/V2324823678/overview