langchain==1.4.0

Changes since langchain==1.3.18

docs(langchain): runnable langchain.mcp examples (#39976)
feat(langchain): langchain.mcp namespace, MCPAdapter (#39939)
perf(anthropic,langchain): omit middleware trace inputs (#40098)
fix(langchain): include model destination in agent tool routing (#38355)
chore(langchain): bump vcrpy test dependency minimum to >=8.2.0 (#39942)