URL context tool not working together with Google Search on Vertex AI

When using the Gemini API, the URL context tool works fine together with Google Search, as documented here:
https://ai.google.dev/gemini-api/docs/url-context

However, on Vertex AI, I'm getting the following error:
```
{'error': {'code': 400, 'message': 'Multiple tools are supported only when they are all search tools.', 'status': 'INVALID_ARGUMENT'}}
```

Test code:
```python
from google import genai
from google.genai import types

def generate():
client = genai.Client(
vertexai=True,
project="XXX",
location="global",
)

model = "gemini-2.5-flash-preview-05-20"
contents = [
types.Content(
role="user",
parts=[
types.Part.from_text(text="""Summarize this page: https://ai.google.dev/gemini-api/docs/url-context"""),
],
),
]
tools = [
types.Tool(url_context=types.UrlContext()),
types.Tool(google_search=types.GoogleSearch()),
]
generate_content_config = types.GenerateContentConfig(
tools=tools,
response_mime_type="text/plain",
)

for chunk in client.models.generate_content_stream(
model=model,
contents=contents,
config=generate_content_config,
):
print(chunk.text, end="")

if __name__ == "__main__":
generate()
```

If I change client to use the Gemini API with an API key, the same code works.