Forward LangChain agent middleware from AgentSpecLoader to create_agent

[spichen]

Add a middleware keyword to the LangGraph adapter's AgentSpecLoader and its underlying AgentSpecToLangGraphConverter. When non-empty, the list is passed verbatim as the middleware= argument of langchain_agents.create_agent inside _create_react_agent_with_given_info. The list is copied defensively on construction and again on forward, so caller-side mutations cannot leak into subsequent conversions.

When the parameter is None or an empty list, create_agent is called with exactly the previous keyword set — no middleware= kwarg is passed — preserving byte-identical behavior for existing users.

The motivating use case is downstream consumers that need to inject middleware such as HumanInTheLoopMiddleware or bespoke state / message-rewriting middleware into the ReAct agent without copy-pasting the entire _create_react_agent_with_given_info method. Middleware is a LangGraph-exposed concept, so the parameter is LangGraph-adapter specific — other adapters have their own AgentSpecLoader classes and are unaffected.

Tests in tests/adapters/langgraph/test_middleware_parameter.py cover: the default (no kwarg forwarded); the empty-list case (also not forwarded); non-empty list forwarded in order; the converter accepting middleware directly without going through the loader; and the list being copied so post-construction mutation of the caller's list has no effect on conversions.

Full tests/adapters/langgraph suite passes under SKIP_LLM_TESTS=1 (68 passed, 71 skipped, 0 failed).