VertexAISearchTool using Google Drive data store does not return results to Dev UI unless streaming is enabled

[andrewlarimer]

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Describe the bug
If you connect to a Vertex AI Search data store using the VertexAISearchTool and query it from an agent in the Dev UI, it will appear as though the interface has frozen (agent response dots repeating infinitely).

The search results have been returned, and you can see them in the Terminal pane where the adk web Dev UI has been run. If you use adk run on the agent instead, you can see the results returned and the agent response. If you enable streaming within the adk web dev ui, you can also see the results return.

But without streaming, it just hangs.

To Reproduce
Steps to reproduce the behavior:

1. Create a vertex AI data store (I used unstructured data in Google Drive)
2. In an agent, add the VertexAISearchTool, like so:

from google.adk.tools import VertexAiSearchTool

# The data_store_id path follows the same format as the datstore parameter
# of google.genai.types.VertexAISearch. View its documentation here:
# https://googleapis.github.io/python-genai/genai.html#genai.types.VertexAISearch

# Create your vertexai_search_tool and update its path below
vertexai_search_tool = VertexAiSearchTool(data_store_id="projects/YOUR_PROJECT_ID/locations/global/collections/default_collection/dataStores/YOUR_DATA_STORE_ID")

3. Add the tool to an agent and ask it questions about the data store using adk run, and adk web with and without streaming.
4. See that without enabling streaming, it will not return a response in the dev UI

Expected behavior
A response grounded on the data store.

[kssrinivasan]

I have seen the exact same behavior. Reported in google/adk-web#65.