Queues: add API to immediately dead-letter a specific message

Cloudflare Queues push consumers currently expose ack/retry controls, and configured DLQs receive messages only after max_retries is exhausted.

**Feature request:** add a push-consumer API to immediately dead-letter a specific message, for example:

```ts
message.deadLetter({ reason?: string })
```

**Use case:** a consumer can classify some failures as terminal/non-retryable, such as programmer bugs, malformed-but-preservable payloads, or invariant failures. Today the choices are either:

- ack the message and lose the payload, or
- retry/fail the message until max_retries so Cloudflare moves it to the configured DLQ, even though retries are known to be useless, or
- manually publish a custom failure envelope to a separate queue and then ack the original.

An immediate dead-letter primitive would let consumers preserve terminal failures in the configured DLQ without wasting retry attempts and without building a parallel application-level DLQ mechanism.

**Relevant docs:**
- https://developers.cloudflare.com/queues/configuration/dead-letter-queues/
- https://developers.cloudflare.com/queues/configuration/batching-retries/

まず、issue にリンクされている Cloudflare Queues の dead-letter と batching/retries のドキュメントを確認し、その後、既存の push-consumer の ack および retry API を調査します。メッセージ単位の dead-letter 操作とオプションの理由がどのように動作すべきかを定義します。完了条件は、終端エラーが retry 試行を消費せず、直ちに設定済みの DLQ に到達することです。

索引モデルが issue の本文から書いたものです。