[Firebase AI] Replace xcodeproj groups with folders in integration tests

[andrewheard]

Updated the Firebase AI Logic integration test app to the Xcode 16 Project Format and enabled "Minimize Project References". Converted the existing groups into folders.

Minimize project file changes, and avoid version control conflicts with buildable folder references.

Convert an existing group to a buildable folder with the “Convert to Folder” context menu item in the Project Navigator. Buildable folders only record the folder path into the project file without enumerating the contained files. This minimizes diffs to the project when files are added and removed, and avoids source control conflicts with your team.
-- Xcode 16 Release Notes

Although this introduces some churn now, it should help us with merge conflicts going forward.

#no-changelog

[paulb777]

LGTM on green