generate_videos() and operations.get() have incompatible auth requirements

## Description

When using the `google-genai` SDK with `GOOGLE_GENAI_USE_VERTEXAI=True` in the environment, `generate_videos()` fails with:

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

However, when using the Developer client with API key auth, `generate_videos()` succeeds but the subsequent `operations.get()` polling call fails with:

> 401 UNAUTHENTICATED: "API keys are not supported by this API. Expected OAuth2 access token or other authentication credentials that assert a principal."

This means there is no single client configuration that supports the full `generate_videos()` → `operations.get()` flow:

| Client Type | `generate_videos()` | `operations.get()` |
|---|---|---|
| `genai.Client(vertexai=True, ...)` | Fails ("Developer client only") | Would work (OAuth2) |
| `genai.Client(api_key=...)` | Works | Fails (401, needs OAuth2) |
| `genai.Client(project=..., location=...)` (no env var) | Works | Works (ADC) |

## The Problem

The third option works, but **only if `GOOGLE_GENAI_USE_VERTEXAI` is NOT set** in the environment. In production environments where this env var is `True` (for other Gemini services like image generation, transcription, etc.), there is no way to create a Developer client with ADC auth for video generation without temporarily mutating the environment variable.

There is no SDK parameter to override the env var behavior (e.g., `vertexai=False` forces API key mode, not ADC).

## Expected Behavior

One of:
1. `generate_videos()` should work with the Vertex AI client (`vertexai=True`)
2. OR `genai.Client(vertexai=False)` should support ADC auth (not just API key)
3. OR a parameter like `genai.Client(vertexai=None)` to explicitly ignore the env var

## Current Workaround

Temporarily removing the env var during client creation:

```python
env_val = os.environ.pop("GOOGLE_GENAI_USE_VERTEXAI", None)
try:
client = genai.Client(project=project, location=location)
finally:
if env_val is not None:
os.environ["GOOGLE_GENAI_USE_VERTEXAI"] = env_val
```

This is fragile and not thread-safe.

## Environment

- `google-genai` >= 1.20.0
- Python 3.12
- Using Veo 3.1 (`veo-3.1-fast-generate-preview`)
- GCP project with service account (ADC via `GOOGLE_APPLICATION_CREDENTIALS`)