[Bug] The a2a protocol layer is not working

### Search before asking

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

### Environment

Mac

### EventMesh version

master

### What happened

Can not transforms JSON-RPC into CloudEvent corretcly. like methods
This is my request:
```
curl -X POST http://127.0.0.1:10105/eventmesh/publish/test-topic \
-H "Content-Type:application/json" \
-H "protocoltype: a2a" \
-H "protocolversion: 2.0" \
-d '{
"jsonrpc": "2.0",
"method": "message/send",
"id": "curl-req-002",
"params": {
"message": {
"role": "user",
"message_id": "msg-456",
"parts": [
{
"kind": "text",
"text": "What is the weather in Shanghai?"
}
]
},
"_topic": "test-topic"
}
}'
```
And this is the message:

### How to reproduce

1. launch EventMesh
2. send a json rpc request
3. check protocol type in message is http rather than a2a

### Debug logs

```Java

```

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct) *

Start by reproducing the request against the /eventmesh/publish/test-topic endpoint with protocoltype set to a2a. Trace how the JSON-RPC request is transformed into a CloudEvent and compare the resulting protocol type with the expected a2a value. Done means the published message preserves a2a instead of being reported as http.

Written by the indexing model from the issue text.