Replace private Agent Engine query SDK request

[AlejandroMorgante]

The Vertex AI Agent Engine query operator currently uses the SDK private _api_client.request() path to call the synchronous {reasoningEngine}:query endpoint. This avoids the SDK private _query response parsing issue seen with google-genai==2.8.0 when the response output type is Any, while keeping URL construction and auth in the SDK.

Reference SDK documentation to check for a supported public replacement:
https://docs.cloud.google.com/python/docs/reference/agentplatform/latest/vertexai._genai.agent_engines.AgentEngines#methods

Follow-up work:

• Replace _api_client.request() with a supported public SDK path for the synchronous Agent Engine query endpoint, either when one is exposed or when the SDK response parsing issue is fixed
• Remove the guardrail that raises when _api_client.request is unavailable
• Keep the operator behavior synchronous; async query-job support should remain separate if added