Cache airline flight database between resets

[benjibc]

Summary

• memoize the flight database in AirlineEnvironment.reset and reuse a deep copy on each reset
• add a regression test that exercises two resets and asserts the database only hits disk once
• provide lightweight type stubs for optional langfuse/langsmith dependencies so pre-commit type checks pass

Testing

• pytest tests/pytest/test_mcp_session_autocreate.py::test_airline_environment_reset_uses_cached_db
• pre-commit run --all-files

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https://chatgpt.com/codex/tasks/task_e_68cc8a9c0c6c83339982b08d0550ea71