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

Start in vertexai/_genai/agent_engines.py around lines 1092-1101 and compare how python_version is placed in the query payload with the update mask. Reproduce the Agent Engine update using the issue's vertexai.Client example; done means the requested Python version is included in the update mask and the Agent Engine version changes.

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