[Bug] Proxy ProducerProcessor throws IndexOutOfBoundsException for empty send message list

### Problem

`ProducerProcessor#sendMessage` reads the first message before validating the request payload:

```java
Message message = messageList.get(0);
```

`buildSendMessageRequestHeader` also reads `messageList.get(0)`. If an internal caller passes a null or empty message list, the method fails with a raw `NullPointerException` or `IndexOutOfBoundsException` instead of a controlled proxy error.

### Expected behavior

`sendMessage` should validate `messageList` before reading the first message and complete the returned future exceptionally with a `ProxyException` that has a clear message.

### Impact

Malformed send requests are harder to diagnose because they surface as generic collection exceptions rather than proxy-level validation failures.

### Suggested fix

- Add an early null/empty guard in `sendMessage`.
- Return a `ProxyException` with a clear message such as `message list is empty`.
- Add regression coverage for an empty message list.

Start with ProducerProcessor#sendMessage and buildSendMessageRequestHeader, focusing on their first access to messageList. Add regression coverage for a null or empty message list and verify that the returned future completes exceptionally with a ProxyException containing a clear message.

Written by the indexing model from the issue text.