Describe the bug

When using Gemini 3 models with ADK's native Gemini integration (not LiteLLM), complex tasks involving multiple tool calls fail with a 400 error stating that function calls are missing thought_signature.

Important: This issue occurs even when using Gemini(model="gemini-3-pro-preview") directly, not LiteLLM. As mentioned in #3627 (comment) , the recommendation is to use ADK's native Gemini integration to avoid thought_signature issues, but this error shows that native integration alone does not solve the problem.

To Reproduce

from google.adk import Agent
from google.adk.models.google_llm import Gemini

agent = Agent(
    name="search_agent",
    model=Gemini(model="gemini-3-pro-preview"), # Using native Gemini, not LiteLLM
    tools=[google_search_tool],
)

# Execute complex task requiring multiple tool calls
response = await agent.ask_stream(
    messages=[Message(role=MessageRole.USER, contents=[
        TextContent(text="Complex task requiring multiple searches")
    ])]
)

Error

400 Bad Request
{
  "error": {
    "code": 400,
    "message": "Unable to submit request because function call `default_api:google_search` in the 5. content block is missing a `thought_signature`. Learn more: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/thought-signatures",
    "status": "INVALID_ARGUMENT"
  }
}

The error occurs in the 5th content block after multiple tool invocations.

Expected behavior

ADK's native Gemini integration should automatically handle thought_signature requirements for function calling. Users should not need to manually manage this Gemini 3-specific requirement.

Environment

• OS: macOS
• Python version: 3.13
• ADK version: Latest
• Are you using LiteLLM: No (using native Gemini class)
• Model: gemini-3-pro-preview

Additional context

• Related: Cannot use Gemini 3 with litellm / vertex AI #3627 (comment) suggests using native Gemini integration, but this issue shows that's not sufficient.