Missing type stub for agent_engines.memories.retrieve() method

# Missing Type Stubs for `client.agent_engines.memories.retrieve()` in Google ADK

## **Description**

The `client.agent_engines.memories.retrieve()` method works correctly at runtime, but it is **missing from the type stubs**. As a result, IDEs and type checkers (Pylance, Pyright, mypy) report errors such as:

```
Cannot access attribute "retrieve" for class "_AgentEngineMemories"
```

The method itself *does* exist and functions properly during execution.

---

## **Environment**

* **google-adk version:** (run: `pip show google-adk`)
* **vertexai version:** (run: `pip show google-cloud-aiplatform`)
* **Python version:** 3.12

---

## **Steps to Reproduce**

```python
import vertexai

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

# Works at runtime, but IDE reports a type error
retrieved_memories = client.agent_engines.memories.retrieve(
name="projects/.../reasoningEngines/...",
scope={"app_name": "test", "user_id": "user123"},
similarity_search_params={"search_query": "*"}
)
```

---

## **Expected Behavior**

The SDK's type stubs should correctly declare the `retrieve()` method so that IDEs and type checkers do not show false errors.

---

## **Actual Behavior**

* Runtime: **works correctly**, method is available and returns results.
* Type checking: fails with errors such as:

* `Cannot access attribute "retrieve" for class "_AgentEngineMemories"`

---

## **Additional Context**

* The implementation exists in `vertexai/_genai/memories.py` (around lines 1182–1233, inside the `Memories` class).
* However, the **type stub files (.pyi)** do not expose this method.
* Other methods under `agent_engines.memories` such as `generate()`, `create()`, `list()`, and `get()` may also require type stub validation.

This issue affects static analysis and IDE tooling but not runtime functionality. A stub update should resolve it.