Describe the bug
When using Google Gemini (Gemini 3 series) as the provider, attempting to use extensions/tools results in a 400 Bad Request error.

The API explicitly complains that the thought_signature is missing from the function call history. According to Gemini documentation, this signature must be preserved and sent back to the API exactly as received during multi-turn function calling loops.

Error Log:

Request failed with status: 400 Bad Request. 
Message: Function call is missing a thought_signature in functionCall parts. 
This is required for tools to work correctly, and missing thought_signature may lead to degraded model performance. 
Additional data, function call default_api:todo__todo_read , position 2.

To Reproduce
Steps to reproduce the behavior:

1. Configure Goose to use the Google Gemini provider (specifically a model like gemini-3-pro-preview or a thinking model).
2. Enable an extension (e.g., the Built-in functionality or a Todo extension as seen in the logs).
3. Prompt the model to perform a task that requires a tool/function call (e.g., "Read my todo list").
4. See error: 400 Bad Request ... missing thought_signature.

Expected behavior
Goose should capture the thought_signature field included in the functionCall or content parts of the Gemini response and pass it back unchanged in the conversation history for subsequent requests. This ensures the tool output validation passes on the API side.

Please provide the following information

• OS & Arch: [e.g. Mac M1 / Ubuntu 22.04 / Windows 11]
• Interface: [UI / CLI]
• Version: [e.g. v1.0.2]
• Extensions enabled: [e.g. Memory, Computer Controller]
• Provider & Model: [e.g. Google – gemini-3-pro-preview]

Additional context
This seems to be a strict validation requirement introduced in newer Gemini models (Gemini 3 Pro).

References from Gemini API Docs:

"As a general rule, if you receive a thought signature in a model response, you should pass it back exactly as received when sending the conversation history in the next turn. When using Gemini 3 Pro, you must pass back thought signatures during function calling, otherwise you will get a validation error (4xx status code)."

"The first functionCall part in each step of the current turn must include its thought_signature. If you omit a thought_signature... the request will fail with a 400 error."

Documentation Link: https://ai.google.dev/gemini-api/docs/thought-signatures

Proposed Solution for Devs:
Ensure the internal handling of functionCall objects or Content parts includes the thought_signature property (encrypted string) and includes it when constructing the message history payload for the next turn.