Async middleware decorator typing fixed

Changes since langchain==1.3.7

release(langchain): 1.3.8 (#38096) style(core,langchain,langchain-classic,partners): replace double backticks in docstrings (#38095) release(core): 1.4.6 (#38061) chore(langchain): add overloads to create_agent (#34309) chore(infra): bump mypy to 2.1 and unify type-check config across the monorepo (#36470) fix(langchain): support async middleware decorator typing (#34584) fix(langchain): tighten structured output model fallbacks (#38042) release(anthropic): 1.4.5 (#38036) hotfix(core): bump lockfile(s) (#38032) refactor(langchain): refactor test_create_agent_tool_validation (#34443)