Feature Parity: Enable File Upload API support for Vertex AI clients

### What you're trying to do

I am looking to use the **File Upload API** within the new `google-genai` SDK while using the Vertex AI backend.

Even though recent blog posts and documentation (like [this one](https://blog.google/innovation-and-ai/technology/developers-tools/gemini-api-new-file-limits/)) treat the File API as a standard feature, it currently appears to be restricted to the Gemini (Developer) client.

Due to specific contractual and compliance requirements between my company and Google Cloud, we are standardized on **Vertex AI**. As a result, having feature parity for file management in the Vertex-enabled SDK is a high priority for our workflow.

### Any error messages you're getting

When attempting to use `client.files.upload` with a Vertex-initialized client, I receive the following error:

> *"This method is only supported in the Gemini Developer client"*

**Reference:** [google/genai/files.py#L194](https://www.google.com/search?q=https://github.com/googleapis/python-genai/blame/af94e536ab606d1795ad90e0c8eeb9c2a850ce6b/google/genai/files.py%23L194)

### Proposed Solution

It would be great to see support for `client.files` methods (upload, get, list, etc.) extended to the Vertex AI backend without having to switch implementation patterns or use separate APIs for file handling.