Fix GoogleModel thinking signature not preserved in multi-turn conversations

[jerry-heygen]

Summary

• Fix regression where ThinkingPart.signature was never populated for Google model responses (introduced in Don't insert empty ThinkingPart when Google response ends in text with thought_signature #3516)
• This caused thinking context to be lost in follow-up requests, as Google requires the thought_signature to be sent back to maintain reasoning continuity
• Google's API returns thought_signature on the part following the thinking content (e.g., TextPart or ToolCallPart), not on the ThinkingPart itself
• The fix applies the signature from the following part back to the preceding ThinkingPart.signature

Problem

After #3516, ThinkingPart.signature was always null. The signature was being stored in provider_details on the TextPart/ToolCallPart, but never preserved on the ThinkingPart.

This is critical because when sending message history back to Google, _content_model_response() reads from ThinkingPart.signature to include the signature on the next part. Without it, thinking context is lost in multi-turn conversations.

Per Google's documentation:

"Signatures are returned from the model within other parts in the response, for example function calling or text parts."
"Always send the thought_signature back to the model inside its original Part."

Solution

Non-streaming path: Track last_thinking_part and when a non-thinking part arrives with thought_signature, apply it back to ThinkingPart.signature.

Streaming path: When a non-thinking part with thought_signature arrives, emit a handle_thinking_delta event with the signature to update the previous thinking part.

Test plan

• All existing Google model tests pass (88 passed, 14 skipped)
• test_google_thought_signature_on_thinking_part verifies round-trip behavior
• Updated snapshots to include signature=IsStr() on ThinkingParts