ModuleNotFoundError: No module named 'a2a.server.apps'

### What happened?

Importing A2AStarletteApplication from a2a.server.apps raises a ModuleNotFoundError.
I encountered this error in **line 3** of the following file from the official samples repository:
[samples/python/agents/helloworld/__main__.py](https://github.com/a2aproject/a2a-samples/blob/main/samples/python/agents/helloworld/__main__.py)

### **The file contains:**
```python
from a2a.server.apps import A2AStarletteApplication
```

### **Error Traceback:**
`ModuleNotFoundError: No module named 'a2a.server.apps'`

---

### **What I Checked :**
- Verified the installed package version using pip show a2a-sdk
- Explored the GitHub repository structure
- Looked inside the server directory in the repository
- Could not find any apps module under [a2a/server](https://github.com/a2aproject/a2a-python/tree/main/src/a2a/server)

### **Environment :**
- OS: Windows
- Python version: 3.14.4
- a2a-sdk version: 1.0.0

---

### Relevant log output

```shell
----> 1 from a2a.server.apps import A2AStarletteApplication

ModuleNotFoundError: No module named 'a2a.server.apps'
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Check the a2a-python repository's src/a2a/server directory to see the actual module structure. Compare the import path in the sample (a2a.server.apps) with the available modules. The fix likely involves updating the import statement in samples/python/agents/helloworld/__main__.py to match the correct module name, such as a2a.server.application. Run the sample after the change to verify the import works.

索引モデルが issue の本文から書いたものです。