[Doc] No A2AProtocolTransportObject in EventMesh SDK

### Search before asking

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

### Documentation Related

https://github.com/apache/eventmesh/blob/master/docs/a2a-protocol/README_EN.md#1-initiate-call-client
```
Usage Guide
1. Initiate Call (Client)
// 1. Construct MCP Request JSON
String mcpRequest = "{" +
"\"jsonrpc\": \"2.0\"," +
"\"method\": \"tools/call\"," +
"\"params\": { \"name\": \"weather\", \"_agentId\": \"weather-agent\" }," +
"\"id\": \"req-001\"" +
"}";

// 2. Send via EventMesh SDK
eventMeshProducer.publish(new A2AProtocolTransportObject(mcpRequest));
```

### Are you willing to submit PR?

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

### Code of Conduct

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

Start with docs/a2a-protocol/README_EN.md at the “Initiate Call (Client)” section and compare its A2AProtocolTransportObject example with the EventMesh SDK APIs. Confirm whether the documentation should reference an existing transport object or whether the missing type is intended; done means the guide no longer references an unavailable class and the example matches the SDK.

Written by the indexing model from the issue text.