Ensure multi-client configurations use new ProxyClient (#1045)

* Validate elicitation responses are dicts

* Ensure advanced MCP features are forwarded through proxy

* Use proxy client in tests

src/fastmcp/client/elicitation.py

@@ -53,6 +53,11 @@ def create_elicitation_callback(
             if not isinstance(result, ElicitResult):
                 result = ElicitResult(action="accept", content=result)
             content = to_jsonable_python(result.content)
+            if not isinstance(content, dict | None):
+                raise ValueError(
+                    "Elicitation responses must be serializable as a JSON object (dict). Received: "
+                    f"{result.content!r}"
+                )
             return MCPElicitResult(**result.model_dump() | {"content": content})
         except Exception as e:
             return mcp.types.ErrorData(

src/fastmcp/client/transports.py

@@ -773,8 +773,6 @@ class MCPConfigTransport(ClientTransport):
     """
 
     def __init__(self, config: MCPConfig | dict):
-        from fastmcp.client.client import Client
-
         if isinstance(config, dict):
             config = MCPConfig.from_dict(config)
         self.config = config
@@ -792,9 +790,9 @@ class MCPConfigTransport(ClientTransport):
             composite_server = FastMCP()
 
             for name, server in self.config.mcpServers.items():
-                server_client = Client(transport=server.to_transport())
                 composite_server.mount(
-                    prefix=name, server=FastMCP.as_proxy(server_client)
+                    prefix=name,
+                    server=FastMCP.as_proxy(backend=server.to_transport()),
                 )
 
             self.transport = FastMCPTransport(mcp=composite_server)