Vertex AI Search Grounding Metadata not included in Gemini 3.5 response

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**Describe the Bug:**
As Gemini 2.5 is getting deprecated, we are forced to move to Gemini 3.5. Our application strictly requires that grounding metadata are included in the model response. We use `VertexAiSearchTool` (data store that indexes text files in a bucket). We infer the URL of the internal documentation from the object URI. Furthermore, we have safeguards in place that reject a response that has no grounding chunks.

**Steps to Reproduce:**
Please provide a numbered list of steps to reproduce the behavior:

1. Use VertexAISearchTool.
2. Use Gemini 3.5 Flash in the EU region.
3. Attempt to extract grounding metadata during an after_model_callback.

**Expected Behavior:**
A clear and concise description of what you expected to happen.

**Observed Behavior:**
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**Environment Details:**

- ADK Library Version: 2.5
- Desktop OS: macOS + Linux
- Python Version (python -V): >=3.13

**Model Information:**

- Are you using LiteLLM: No
- Which model is being used: gemini-3.5-flash, gemini-3.5-flash-lite
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**Regression:**
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**Additional Context:**
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**Minimal Reproduction Code:**
Instantiate an agent with the appropriate tool

**How often has this issue occurred?:**

- Always (100%)