`thought_signature` invalid when pass back to sdk

[wjw99830]

Environment details

• Programming language: python
• OS: linux
• Language runtime version: 3.13
• Package version: 1.65.0

Steps to reproduce

1. Generate function call and thought_signature by generate_content_stream
2. Pass thought_signature back to next request
3. Error: gemini.RequestGoogleModeData.Contents: []*genai.Content: genai.Content.Role: Parts: []*genai.Part: genai.Part.ThoughtSignature: decode base64: illegal base64 data at input byte 40, error found in #10 byte of ...|l9oZl-zw="}],"role":|..., bigger context ...|XKZTIhg4Gq5vvk9xV7By9Kc7Rpdq-VMhvheurUcWl9oZl-zw="}],"role":"model"},{"parts":[{"functionResponse":{|...

The / and + are replaced to _ and - when passing back to sdk.