Add Gemini Thought Signatures to LiteLLM Vertex AI implementation

[RajeshThallam]

Adding Gemini Thought Signatures to LiteLLM Vertex AI

This PR adds Gemini Thought Signatures to LiteLLM Vertex AI implementation.

======================================= test session starts ========================================
platform linux -- Python 3.13.3, pytest-8.4.1, pluggy-1.6.0
rootdir: /usr/local/google/home/rthallam/litellm_fork
configfile: pyproject.toml
plugins: anyio-4.8.0, ddtrace-2.19.0
collected 26 items                                                                                 

tests/test_litellm/llms/vertex_ai/gemini/test_vertex_and_google_ai_studio_gemini.py ........ [ 30%]
..................                                                                           [100%]

======================================== 26 passed in 2.59s ========================================

Relevant issues

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• I have added a screenshot of my new test passing locally
• My PR passes all unit tests on make test-unit
• My PR's scope is as isolated as possible, it only solves 1 specific problem

Type

🆕 New Feature
🐛 Bug Fix
🧹 Refactoring
📖 Documentation
🚄 Infrastructure
✅ Test

Changes

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
litellm	Ready	Preview	Comment	Aug 25, 2025 4:52am

[CLAassistant]

All committers have signed the CLA.