A2A AgentSkill examples field

I have a question. The example of agent card defines the format as json. Why is the received JSON-RPC a text?

```java
Agent Card
AgentSkill skill = new AgentSkill(
"ai-whiteboard-id",
"create a whiteboard",
"use it for create a whiteboard, input must be {\"skillId\":\"ai-whiteboard-id\",\"param\":{\"name\":\"whiteboard_1 name\"}}",
List.of("whiteboard"),
List.of(
"input: {\"skillId\":\"ai-whiteboard-id\",\"param\":{\"name\":\"whiteboard_1 name\"}}",
"input: {\"skillId\":\"ai-whiteboard-id\",\"param\":{\"name\":\"whiteboard_2 name\"}}"),
List.of("application/json"),
List.of("application/json"));
```

a2a interface
2025-07-17T15:00:42.579Z DEBUG 2555609 --- [a2a-server] [nio-9080-exec-4] com.google.a2a.server.A2AController : Received JSONRPCRequest:
```json
{
"id" : "736fd61a-ac95-4c2e-bc74-3cbc199e5ad5",
"jsonrpc" : "2.0",
"method" : "message/send",
"params" : {
"configuration" : {
"acceptedOutputModes" : [ "text", "text/plain", "image/png" ]
},
"message" : {
"contextId" : "17dff8d0-a5d6-4368-92e6-d88bc27da1ad",
"kind" : "message",
"messageId" : "1df29cc1-9fba-4bfa-878e-fb9fee1ae8ed",
"parts" : [ {
"kind" : "text",
"text" : "create a whiteboard named testa2a001"
} ],
"role" : "user"
}
}
}
```

Look at the AgentSkill class definition and how examples are serialized/deserialized. Compare the Agent Card example format with the actual JSON-RPC request received in A2AController. Check if there's a mismatch in data types (JSON vs text) in the examples field. Run tests related to AgentSkill or A2A message handling to see the current behavior.

索引モデルが issue の本文から書いたものです。