Function call_tool_json 

pub async fn call_tool_json(
    handler: &dyn DynToolHandler,
    name: &str,
    args: Map<String, Value>,
    ctx: &Context<'_>,
) -> Result<Value, McpError>

Run a tool and return its CallToolResult as JSON (the tasks/result payload shape). Shared by the stdio runtime and the HTTP adapters.