`get_version` returns latest prompt content instead of specified version content

### Description

**Bug Description**

The `get_version` method in `Prompts` class (and `AsyncPrompts`) always returns the latest prompt content regardless of which `version_id` is specified. This is because the method creates the `Prompt` object from `prompt_dataset_resource` (which contains the latest prompt data) instead of `prompt_version_resource` (which contains the version-specific data).

**Steps to Reproduce**

1. Create a prompt with initial content (e.g., "Version 1 content")
2. Create a new version with different content (e.g., "Version 2 content")
3. Repeat step 2 to create multiple versions
4. Call `get_version(prompt_id="...", version_id="1")` to retrieve version 1
5. The returned prompt contains "Version 2 content" (latest) instead of "Version 1 content"

**Expected Behavior**

`get_version` should return the prompt content corresponding to the specified `version_id`.

**Actual Behavior**

`get_version` always returns the latest prompt content, ignoring the `version_id` parameter for the prompt data.

**Root Cause**

In `vertexai/_genai/prompts.py`, the `get_version` method creates the `Prompt` object from `prompt_dataset_resource`:

https://github.com/googleapis/python-aiplatform/blob/47be102c3d27170fe605e9aec69d9d97d9f305ee/vertexai/_genai/prompts.py#L1145-L1175

**Suggested Fix**

The `Prompt` object should be created from `prompt_version_resource` instead:

```python
# Proposed fix
prompt_dataset_resource = self._get_dataset_resource(name=prompt_id)
prompt_version_resource = self._get_dataset_version_resource(
dataset_id=prompt_id,
dataset_version_id=version_id,
)

prompt = _prompt_management_utils._create_prompt_from_dataset_metadata(
prompt_version_resource, # ← Use version-specific resource
)

prompt._dataset = prompt_dataset_resource
prompt._dataset_version = prompt_version_resource

return prompt
```

**Affected Code**

- `Prompts.get_version` (sync version): lines 1145-1175
- `AsyncPrompts.get_version` (async version): lines 2311-2341

**Environment**

- vertexai SDK version: 1.130.0
- Python version: 3.13.2

从 vertexai/_genai/prompts.py 中的 Prompts.get_version 和 AsyncPrompts.get_version 开始，使用所引用的行范围和复现步骤比较 dataset 资源和版本资源。完成标准是：请求较旧版本时，在同步和异步方法中都会返回该版本自身的 prompt 内容，而不是最新内容。

由索引模型根据 Issue 内容生成。