[AI] Add wrapper for FoundationModels.GenerationOptions

[andrewheard]

Added a thin wrapper class FirebaseAI.GenerationOptions to facilitate usage of FoundationModels.GenerationOptions on iOS 15+, macOS 10.15+, tvOS 15+, watchOS 8+ and visionOS 1+ in #16043. The wrapped FoundationModels.GenerationOptions is only available on iOS/macOS/visionOS 26+.

#no-changelog

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[danger-firebase-ios]

	1 Warning
⚠️	New public headers were added, did you remember to add them to the umbrella header?

Generated by 🚫 Danger

[gemini-code-assist]

Code Review

This pull request introduces GenerationOptions and SamplingMode types as thin wrappers for FoundationModels to control model response generation, along with corresponding unit tests. Feedback focuses on ensuring consistent platform support by adding visionOS 26.0 to several @available attributes, matching the availability of the underlying FoundationModels types.

[paulb777]

Please clear CI issues.