[Feature] Add AsyncRpcResult test sample in dubbo-samples

### Pre-check

- [x] I am sure that all the content I provide is in English.

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/dubbo/issues?q=is%3Aissue) and found no similar feature requirement.

### Apache Dubbo Component

Java Samples (apache/dubbo-samples)

### Descriptions

### Background
`AsyncRpcResult` is the key class for asynchronous calls in Dubbo Java SDK, but the official dubbo-samples repository does not contain any test case or example to demonstrate its correct usage. This makes it hard for users to understand how to:

1. Obtain the result via AsyncRpcResult after an async call.
2. Handle timeout and exceptions properly.
3. Ensure compatibility across Dubbo versions.

### Proposal
Create a new sub-module `dubbo-samples-async-result` under `2-advanced/` with:

- A provider that exposes an async service.
- A consumer that uses AsyncRpcResult to get the response and handle timeout/exception.
- Docker-based integration tests similar to existing samples (`dubbo-samples-annotation`, etc.).

### Acceptance Criteria
- [ ] New module builds and passes `./test/run-tests.sh 2-advanced/dubbo-samples-async-result`.
- [ ] README explains how to run the demo and the key points of AsyncRpcResult.
- [ ] No breaking changes to existing samples.

### Volunteer
I’m willing to submit a PR once the feature request is accepted.

### Related issues

_No response_

### Are you willing to submit a pull request to fix on your own?

- [ ] Yes I am willing to submit a pull request on my own!

### Code of Conduct

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