bug(ai): Error sending prompt to AI: TypeError: Cannot read properties of undefined (reading 'some')

### Operating System

Windows

### Environment (if applicable)

Chrome

### Firebase SDK Version

11.9.1

### Firebase SDK Product(s)

AI

### Project Tooling

Angular

### Detailed Problem Description

I ran into this error once when sending a request to Gemini 2.5 Pro via `generateContentStream`:
```dart
Error sending prompt to AI: TypeError: Cannot read properties of undefined (reading 'some')
```


https://github.com/firebase/firebase-js-sdk/blob/86155b3c8f3974f8d777232625108c14f924e035/packages/ai/src/googleai-mappers.ts#L179

Perhaps `candidate.content?.parts.some(` should be `candidate.content?.parts?.some(`.

### Steps and code to reproduce issue

Most requests don't seem to run into this issue, but one request did.

Replacing `parts.some` with `parts?.some` in [googleai-mappers.ts](https://github.com/firebase/firebase-js-sdk/blob/86155b3c8f3974f8d777232625108c14f924e035/packages/ai/src/googleai-mappers.ts#L179) should resolve the issue.