Tool middleware in MoonBit

For typed middleware, annotate an empty struct and implement async methods whose first parameter is the generated underlying. Echo must be a same-package #derive.tool declaration; see golem-define-tool-moonbit.

#derive.tool_middleware("echo-policy", presented="Echo")
struct EchoPolicy {}
 
pub async fn EchoPolicy::echo(
  underlying : EchoUnderlying,
  value : String,
) -> Result[String, @toolMiddleware.ToolInvokeError[@tool.NoToolError]] {
  underlying.echo(value)
}

Set expected="OtherTool" for an adapter and translate its values and errors. Use #derive.universal_tool_middleware on an async free function only when arbitrary tool metadata and raw carriers are required. MoonBit async functions use no await keyword. The underlying tool is valid only during this middleware invocation and may be called zero, one, or multiple times. Raw input/result carriers and owned stream or permission-card handles are take-once: do not reuse a transferred carrier or let invocation-scoped resources escape.