ReasoningEngineSpec missing effectiveIdentity field causes ParseError when creating Agent Engines

## Environment

- **google-cloud-aiplatform version:** 1.136.0
- **google-adk version:** 1.24.0
- **Python version:** 3.12
- **OS:** Ubuntu Linux (GCE VM)

## Description

When creating a new Reasoning Engine using `vertexai.agent_engines.create()`, the API returns a response containing an `effectiveIdentity` field that is not defined in the local protobuf definitions, causing a `ParseError`.

The `effectiveIdentity` field is documented in the [REST API reference](https://cloud.google.com/vertex-ai/generative-ai/docs/reference/rest/v1/projects.locations.reasoningEngines#ReasoningEngineSpec) as an output-only field, but the Python SDK's proto definitions don't include it.

## Steps to Reproduce

```python
import vertexai
from vertexai import agent_engines

vertexai.init(project="my-project", location="us-central1")

# This triggers the error when parsing the API response
engine = agent_engines.create(display_name="test_engine")
```

## Error Message

```
Failed to parse spec field: Message type "google.cloud.aiplatform.v1.ReasoningEngineSpec" has no field named "effectiveIdentity" at "Operation.response.spec".
Available Fields(except extensions): "['sourceCodeSpec', 'serviceAccount', 'packageSpec', 'deploymentSpec', 'classMethods', 'agentFramework']".
```

## Expected Behavior

The SDK should be able to parse the API response without errors. The `effectiveIdentity` field should be added to the `ReasoningEngineSpec` proto definition.

## Additional Context

This is similar to:
- #5176 - `SpeechConfig` missing `languageCode`
- #4995 - `GenerationConfig` missing `responseModalities`

The pattern is the same: the API returns fields that the SDK's protobuf definitions don't yet support.

首先定位 ReasoningEngineSpec 的 Python protobuf 定义，并使用 vertexai.agent_engines.create() 示例重现解析失败。将其字段与 REST API 参考进行比较，然后验证包含 effectiveIdentity 的响应能够成功解析，并确认相关测试通过。

由索引模型根据 Issue 内容生成。