[Bug]: Check presence of required fileds on server side

[guglielmo-san]

The current server implementation for different protocols does not confirm the presence of required fields in request params.

Expected behavior is specified in 5.7. Field Presence and Optionality.

[zeroasterisk]

Specific repro for Message.message_id (REQUIRED field):

The spec defines Message.message_id as [(google.api.field_behavior) = REQUIRED]. Sending a message without messageId should return -32602 InvalidParams, but the server accepts it and processes the task.

curl -X POST http://localhost:9997/ -H 'Content-Type: application/json' -d '{
  "jsonrpc": "2.0",
  "method": "SendMessage",
  "id": "1",
  "params": {
    "message": {
      "role": "ROLE_USER",
      "parts": [{"text": "no messageId here"}]
    }
  }
}'
# Returns: 200 OK with task result — should return -32602

Expected: {"error": {"code": -32602, "message": "Invalid params: messageId is required"}}

Actual: Task is created and processed successfully.

The same applies to role and parts which are also REQUIRED in the proto.

Found while running the A2A TCK against a v1.0 SDK agent: tests/optional/features/test_invalid_business_logic.py::test_missing_required_message_fields fails with this exact behavior.