include_thoughts=False does not suppress thought parts for image gen models on Vertex AI

## Description

When using `gemini-3.1-flash-image-preview` (NB2) on **Vertex AI** with `ThinkingConfig(include_thoughts=False)`, thought parts (both text and images) are still returned in the response. The `include_thoughts=False` setting has no effect — all thought text and interim "thought images" (drafts) remain in the response.

The `part.thought` flag IS correctly set to `True` on thought parts, so client-side filtering is possible. But per the [docs](https://ai.google.dev/gemini-api/docs/image-generation#thinking-levels), `include_thoughts=False` should suppress thought parts from the response entirely.

## Environment

- SDK: `google-genai==1.70.0`
- API: Vertex AI
- Model: `gemini-3.1-flash-image-preview`
- Platform: Python 3.10

## Steps to Reproduce

```python
from google import genai
from google.genai import types

client = genai.Client(vertexai=True, project="YOUR_PROJECT", location="us-central1")

response = client.models.generate_content(
model="gemini-3.1-flash-image-preview",
contents="Draw a red car",
config=types.GenerateContentConfig(
response_modalities=["TEXT", "IMAGE"],
thinking_config=types.ThinkingConfig(include_thoughts=False),
),
)

for i, part in enumerate(response.candidates[0].content.parts):
print(f"[{i}] thought={part.thought}, has_text={bool(part.text)}, has_image={bool(part.inline_data)}")
```

## Expected Behavior

Only non-thought parts should be returned (the final image):

```
[0] thought=None, has_text=False, has_image=True
```

## Actual Behavior

All thought parts (text and interim draft images) are still present:

```
[0] thought=True, has_text=True, has_image=False # thinking text
[1] thought=True, has_text=True, has_image=False # thinking text
[2] thought=True, has_text=True, has_image=False # thinking text
[3] thought=True, has_text=True, has_image=False # thinking text
[4] thought=True, has_text=False, has_image=True # draft image (thought image)
[5] thought=True, has_text=True, has_image=False # self-critique text
[6] thought=None, has_text=False, has_image=True # final image
```

`thoughts_token_count: 799` — thinking tokens are billed regardless (documented), but the parts themselves should be hidden from the response when `include_thoughts=False`.

## Additional Context

- This only affects **Vertex AI**. On the Developer API, NB2 has no thinking at all (`thoughts_token_count` is `None`).
- `gemini-3-pro-image-preview` rejects `ThinkingConfig` entirely with a 400: "Thinking_config.include_thoughts is only enabled when thinking is enabled" — despite reporting `thoughts_token_count` (~220-240) in every response. Pro's thinking is locked to HIGH and not configurable.
- The `part.thought` boolean is correctly set, so client-side filtering works as a workaround.