feat: in-memory transport support

[wkpark]

In-memory transport is officially supported by FastMCP v2. However, FastMCP v1 already supports in-memory transport.

This is my attempt to support "In-memory" transport in the langchain-mcp-adapters.

config example

    client = MultiServerMCPClient(
        {
            "math": {
                "command": "python3",
                "args": [math_server_path],
                "transport": "stdio",
            },
            "weather": {
                "server": weather_server_module.mcp, # mcp instance
                "transport": "in_memory",
            },
        },
    )
    # Check that we have tools from both servers
    all_tools = await client.get_tools()

See also:

• Add in-memory transport modelcontextprotocol/python-sdk#25
• https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/shared/memory.py
• Add documentation about testing modelcontextprotocol/python-sdk#1426
• Ensure client is only initialized once jlowin/fastmcp#758