support FileContent deserialization

[flycloud-hz]

What's changed?

In a2a server request with file part could not be deserialized correctly.

here is the error message

11:06:43.197 [main] ERROR io.github.a2ap.core.client.impl.DefaultA2AClient - JSON-RPC error when sending message: code=-32602, message=Invalid params, data=Cannot construct instance of `io.github.a2ap.core.model.FileContent` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.github.a2ap.core.model.MessageSendParams["message"]->io.github.a2ap.core.model.Message["parts"]->java.util.ArrayList[0]->io.github.a2ap.core.model.FilePart["file"])
11:06:43.198 [main] ERROR io.github.a2ap.core.client.impl.DefaultA2AClient - Error sending message to system-cloud: Invalid params

This change includes annotations to support concrete FileContent subclasses.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Any Else Note

[flycloud-hz]

@tomsun28 Could you please help review this? Thanks.

[tomsun28]

👍