LangChain integration: add CascadeAgent loops + domain policy routing

[saschabuehrle]

Summary

• add first-class LangChain CascadeAgent wrappers for Python and TypeScript with closed tool loops
• add per-domain policy routing in both wrappers (quality threshold override, force verifier, direct to verifier)
• add ecosystem fixtures/tests for LangGraph, LangSmith metadata path, and LangServe route registration

Validation

• python3 -m pytest -q cascadeflow/integrations/langchain/tests/test_langchain_agent_loop.py cascadeflow/integrations/langchain/tests/test_langchain_integration_features.py cascadeflow/integrations/langchain/tests/test_langchain_products_e2e.py
• pnpm -C packages/langchain-cascadeflow test
• pnpm -C packages/langchain-cascadeflow typecheck
• pnpm -C packages/langchain-cascadeflow build

Notes

• integration-only changes; no core cascade engine changes