`models.get()` does not resolve alias to underlying model version on Vertex AI

## Description

When calling `client.models.get(model="gemini-flash-latest")` on Vertex AI (`location=global`), the response only echoes the alias name back (`publishers/google/models/gemini-flash-latest`) with `version: "default"`. There is no field that reveals the actual resolved model version.

Similarly, `GenerateContentResponse.model_version` just returns `"gemini-flash-latest"`.

This makes it impossible to:
1. Know which model you're actually calling in production
2. Pin to a specific version before an alias rotates
3. Debug behavioral differences between alias and named models

## Empirical evidence

We ran side-by-side tests (temperature=0, no thinking, identical prompts) comparing `gemini-flash-latest` against known model versions on Vertex AI (`location=global`):

| Test (7 deterministic prompts) | Exact output hash match with `gemini-flash-latest` |
|---|---|
| `gemini-2.5-flash` | **5/7** |
| `gemini-3-flash-preview` | **4/7** |

`gemini-flash-latest` produces **byte-different output** from both named models on several prompts, making it impossible to determine which model it actually resolves to.

According to the [changelog](https://ai.google.dev/gemini-api/docs/changelog) (Jan 21, 2026), `gemini-flash-latest` was switched to `gemini-3-flash-preview` on the Google AI API — but this does not appear to match Vertex AI behavior.

## Expected behavior

`models.get()` should return a `resolved_model` or `base_model` field with the actual underlying model identifier (e.g., `gemini-2.5-flash-20250617`), so users can:
- Audit what they're running in production
- Pin to the resolved version before an alias rotates
- Report accurate model versions in bug reports

## Reproduction

```python
from google import genai

client = genai.Client(vertexai=True, project="YOUR_PROJECT", location="global")

# models.get returns no resolution info
info = client.models.get(model="gemini-flash-latest")
print(info.name) # "publishers/google/models/gemini-flash-latest"
print(info.version) # "default"
# No resolved_model, base_model, or similar field

# Response metadata also just echoes the alias
resp = client.models.generate_content(
model="gemini-flash-latest",
contents="hello",
)
print(resp.model_version) # "gemini-flash-latest" — not the actual model
```

## Additional context

- **Platform**: Vertex AI (not Google AI API)
- **Location**: `global`
- **SDK**: `google-genai` (Python)
- **Use case**: Production conversational AI agent using Google ADK with heavy tool calling. We rely on `gemini-flash-latest` for its quality but cannot pin it, and the documented alias rotation schedule appears to only apply to the Google AI API, not Vertex AI.