chore: pin LangChain providers and remove unused experimental stack

[tdpham2]

Summary

• exact-pin the five LangChain provider integrations used by ChemGraph
• remove the unused langchain-experimental dependency and its otherwise-unneeded langchain-community/langchain-classic chain
• keep pyproject.toml and environment.yml aligned
• leave the LangChain/LangGraph runtime, MCP stack, and local-Argo behavior unchanged

Related issues

N/A

Type of change

• Bug fix
• New feature
• Docs
• Chore / refactor / CI

How was this tested?

• audited tracked source, tests, docs, examples, and scripts for langchain_experimental usage (manifest declarations only)
• Python 3.10-3.13 resolver dry runs against the exact runtime and provider pins
• clean editable install: verified langchain_experimental, langchain_community, and langchain_classic are absent
• python -m pip check
• ruff check .
• pytest tests/test_openai_model_normalization.py -q (5 passed)
• pytest tests/ -k "not tblite" (287 passed, 28 skipped, 2 deselected)
• python -m build
• clean wheel install, ChemGraph/provider imports, dependency-absence assertions, and pip check

Checklist

• Branched off the latest main and targets main
• PR is focused on a single logical change
• ruff check . passes
• pytest tests/ -k "not tblite" passes
• Added/updated tests for the change (packaging-only change; existing provider and local-Argo tests pass against the resolved environment)
• Updated docs / README for any user-facing change (no user-facing behavior change)