Python version not added to the Agent Engine update query mask

#### Environment details

- OS type and version: Mac OS 26.1
- Python version: 3.12
- pip version: 25.2
- `google-cloud-aiplatform` version: 1.129.0

#### Steps to reproduce

1. Use `vertexai.Client(project=..., location=...).agent_engine.update(agent=..., config={'python-version': ...})`.
2. The python version is not added to the update mask for the update query to aiplatform.
3. The python version used by the agent engine is not updated.

In the code, the `python_version` package spec is set in the query payload but not the in the `update_mask`.
https://github.com/googleapis/python-aiplatform/blob/3eb38bf1f566545f5a6aba16b356cb2e4c148992/vertexai/_genai/agent_engines.py#L1092-L1101