[Bug]: a2a-java client sends message with empty task ID and context ID

Revise el código de serialización de mensajes del A2A Java client, probablemente en una clase que se encarga de construir solicitudes JSON-RPC. Encuentre dónde se establecen los campos taskId y contextId. Asegúrese de que se omitan los campos opcionales vacíos. Pruebe enviando un mensaje y verificando que la carga útil JSON excluya las cadenas vacías. Compruebe el manejo de respuestas del .Net agent para IDs vacíos.

What happened?

While investigating errors reported by https://darrelmiller.github.io/agentbin/index.html, I found out that messages sent by the A2A Java client contains empty taskId and contextId.
They look like (verbatim):

curl -sN -X POST https://agentbin.greensmoke-1163cb63.eastus.azurecontainerapps.io/spec \
    -H "Content-Type: application/json" \
    -H "Accept: text/event-stream" \
    -d '{
  "jsonrpc": "2.0",
  "id": "839ffa13-d966-462c-88a2-136805189343",
  "method": "SendStreamingMessage",
  "params": {
    "tenant": "",
    "message": {
      "messageId": "'$(uuidgen | tr '[:upper:]' '[:lower:]')'",
      "taskId": "",
      "contextId": "",
      "role": "ROLE_USER",
      "parts": [
        {
          "text": "task-lifecycle process this",
          "metadata": {},
          "filename": "",
          "mediaType": ""
        }
      ],
      "metadata": {},
      "extensions": [],
      "referenceTaskIds": []
    },
    "configuration": {
      "acceptedOutputModes": [],
      "returnImmediately": false
    },
    "metadata": {}
  }
}'

I think the .Net remote agent should reject these messages (or ignore the empty taskId) but it replies with responses such as

data: {"jsonrpc":"2.0","id":"839ffa13-d966-462c-88a2-136805189343","result":{"task":{"id":"","contextId":"","status":{"state":"TASK_STATE_SUBMITTED","timestamp":"2026-03-26T12:06:00.4665426+00:00"}}}}

which are not deserialised by the a2a-java SDK due to the empty task.id.

The a2a-java client should omit optional fields that are not set when sending messages.
If I remove the taskId: "" field from the reproducer, the .Net agent replies with a generated taskId as expected.

Relevant log output

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