ci: route integration tests through LangSmith LLM Gateway (#697)

## Summary
- Add a shared Vitest setup that remaps Anthropic/OpenAI/etc. clients
onto the LangSmith LLM Gateway when `LANGSMITH_GATEWAY_KEY` is present.
- Wire that setup into all package int-test Vitest configs.
- Pass `LANGSMITH_GATEWAY_KEY` into the CI integration-test job (falls
back to direct provider keys if unset).