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Contribution guide.

Describe the bug
Our agent includes several VertexAiSearchTool tools, we pass data_store_id there. One of the stores is a structured data store. After the 1.16.0 release, the agent invocation returns the following error (from the discovery_engine_search):

status: "error"
error_message: "400 `content_search_spec.search_result_mode` must be set to SearchRequest.ContentSearchSpec.SearchResultMode.DOCUMENTS when the engine contains structured data store."

To Reproduce
Please share a minimal code and data to reproduce your problem.
Steps to reproduce the behavior:
Run any agent that includes VertexAiSearchTool with structured data store with adk web locally to reproduce the issue.

Expected behavior
200 llm response

Screenshots

Desktop (please complete the following information):

• OS: [e.g. macOS, Linux, Windows]
• Python version(python -V):
• ADK version(pip show google-adk): 1.16.0

Model Information:

• Are you using LiteLLM: Yes/No
• Which model is being used(e.g. gemini-2.5-pro)

Additional context
Add any other context about the problem here.