gptel-gemini: Support thoughtSignature in tool calls

[surenkov]

This PR updates the Gemini backend to preserve thoughtSignature fields, which is mandatory for Gemini 3 Pro and recommended for Gemini 2.5.

Gemini reasoning models return an encrypted thoughtSignature alongside text or function calls to maintain reasoning context in their stateless API. Per Gemini API specifications:

When using Gemini 3 Pro, you must pass back thought signatures during function calling, otherwise you will get a validation error (4xx status code).

Previously, gptel-gemini discarded this signature. This change ensures the signature is retained and sent back in the conversation history for subsequent turns, preventing API errors with reasoning models.

Changes:

• Response Parsing:
  • The thoughtSignature is extracted and attached to the tool-use plist if a functionCall is present.
  • For text responses, the thoughtSignature is stored as a 'gptel-gemini-thought-signature text property on the response content in the buffer.
• History Extraction:
  • The signature is retrieved from the text property for model text responses and from the tool call plist for model function calls.
  • The retrieved thoughtSignature is then included in the message structure sent back to the Gemini API.

Closes #1146