Function call is missing a thought_signature in functionCall parts from Gemini API (model: gemini-3-flash-preview)

**Describe the bug**

Function call is missing a thought_signature in functionCall parts. Status: INVALID_ARGUMENT
When the response contains candidate with extra parts with though_signature like:

```json
"candidates": [
{
"content": {
"parts": [
{
"functionCall": {
...
}
},
{
"text": "",
"thoughtSignature": "...thoughtSignature here..."
}
],
"role": "model"
},
"finishReason": "STOP",
"groundingMetadata": {
"retrievalMetadata": {},
"searchEntryPoint": {
"renderedContent": "..."
},
"webSearchQueries": [
...
]
}
}
],
```

**To Reproduce**

This case happens occasionally, and this case is not included in the document of thoughtSignature in `https://docs.cloud.google.com/vertex-ai/generative-ai/docs/thought-signatures` yet sometimes it returns such response

**Expected behavior**

Set the thoughtSignature in the first part of functionCall event with an extra candidate part and no thoughtSignature in functionCall part.

**Screenshots**

**Version:**

gemini@0.1.28

**Environment:**

The output of `go env`.

**Additional context**

Add any other context about the problem here.

No repository file, test, or entry point is named. Start by reproducing the intermittent response with gemini-3-flash-preview and inspect how Gemini API candidate parts are handled in version 0.1.28. Done means functionCall events receive the thoughtSignature from the extra candidate part and no INVALID_ARGUMENT error occurs.

Written by the indexing model from the issue text.