LangChain 1.4.0 alphas build first-party MCP adapter

LangChain 1.4.0a3 (Sep 1) and a4 (Sep 2) extend the new langchain.mcp namespace: list_tools with cache_mode caching (SEP-2549), tool conversion renamed to as_langchain_tool, tool metadata grouped under an mcp namespace, and a required fastmcp>=4.0.0 (a4 fixes 4.0.1 compat and gates interrupt routing on the negotiated protocol era). The alpha series (a1-a2, Aug 27-28) introduced MCPAdapter, turning any MCP server into LangChain tools with multi-server auth, protocol-era negotiation, and elicitation surfaced as LangGraph interrupts - porting and superseding langchain-mcp-adapters.