[firebaseai] Navigation Title Not Rendering in iOS 26 Beta for Specific UI Layouts

Several example screens in the Firebase AI quickstart project fail to display navigation titles when running on iOS 26 beta simulator.

Steps to Reproduce
1. Open Firebase AI iOS Quickstart project in Xcode 26.0 beta2
2. Run on iOS 26 beta simulator. For example, iphone16 pro 26.0
3. Navigate to "generate content" example and observe missing navigation title
4. Compare with "Chat" example which works correctly

Working Examples (navigation title displays correctly):
- ✅ ChatExample
- ✅ FunctionCallingExample

Broken Examples (navigation title missing):
- ❌ GenerativeAITextExample
- ❌ GenerativeAIMultimodalExample
- ❌ ImagenScreen

In the `GenerativeAIMultimodalExample/Screens/PhotoReasoningScreen.swift`, I tried to move the MultimodalInputField under the ScrollViewReader in the VStack. The modified code structure is as below, which solved this issue. But I don't know why.

```swift
VStack {
ScrollViewReader { scrollViewProxy in
List { /* content */ }
}
MultimodalInputField(text: $viewModel.userInput, selection: $viewModel.selectedItems)
}
```
Maybe it's a bug for new iOS system. Should we wait for the final release of iOS 26 or reorganizes UI structure in affected examples?