fix(apple-ai): wrap FoundationModels code in canImport check to fix CI build

[peterfriese]

This PR wraps all new Apple Intelligence FoundationModels code pathways and navigation routes inside #if canImport(FoundationModels) compile-time checks. This allows the project to build successfully on older SDK targets (like Xcode 16.4/iOS 18.5) where the FoundationModels framework does not exist.

[gemini-code-assist]

Code Review

This pull request integrates Apple's local Foundation Models (Apple Intelligence) with automatic cloud fallback into the Firebase AI Example app, introducing new screens and view models for 'Hybrid AI' and 'Vision ID' features, along with robust ML asset error handling. The code review provides several valuable recommendations to improve the implementation: using the .task(id:) modifier in VisionIDView to avoid race conditions during image loading, guarding against task cancellation in HybridAIViewModel and VisionIDViewModel to prevent redundant cloud fallback requests, clearing the error state when the error sheet is dismissed in FoundationModelsContainer, and hiding the default scroll content background of TextEditor in HybridAIView to correctly display the custom background color.

[gemini-code-assist]

Spawning an unstructured Task inside .onChange to load the image data asynchronously can lead to a race condition. If the user quickly selects multiple images in succession, the tasks may complete out of order, resulting in the wrong image being displayed and processed. Using the .task(id:) modifier is the idiomatic SwiftUI way to handle asynchronous work triggered by state changes, as it automatically cancels the previous task when the ID changes.

      .task(id: photosPickerItem) {
        guard let photosPickerItem else { return }
        if let data = try? await photosPickerItem.loadTransferable(type: Data.self),
           let image = UIImage(data: data) {
          viewModel.selectedImage = image
          viewModel.identifySelectedImage()
        }
      }

[gemini-code-assist]

If the local model task is cancelled (for example, if the user cancels the active task or starts a new summarization), the respond call will throw a cancellation error. Catching this error and proceeding directly to the cloud fallback is incorrect, as it will trigger an unwanted cloud API request for a cancelled operation. Guarding against cancellation at the start of the catch block prevents this resource waste.

        } catch {
          guard !Task.isCancelled else { return }
          // Fall back to cloud model if local model fails
          if error.isMLAssetUnavailable {
            print("Local ML assets unavailable. Falling back to cloud model...")
          } else {
            print(
              "Local model failed: \(error.localizedDescription). Falling back to cloud model..."
            )
          }
        }

[gemini-code-assist]

If the local model task is cancelled, the respond call will throw a cancellation error. Catching this error and proceeding directly to the cloud fallback is incorrect, as it will trigger an unwanted cloud API request for a cancelled operation. Guarding against cancellation at the start of the catch block prevents this resource waste.

        } catch {
          guard !Task.isCancelled else { return }
          if error.isMLAssetUnavailable {
            print(
              "Local ML assets unavailable for Vision ID. Falling back to cloud model..."
            )
          } else {
            print(
              "Local model failed for Vision ID: \(error.localizedDescription). Falling back to cloud model..."
            )
          }
        }

[gemini-code-assist]

When the error sheet is dismissed, viewModel.error is not cleared. This leaves stale error state in the view model. Clearing the error on dismiss ensures that the view model state stays in sync with the UI presentation.

Suggested change

	.sheet(isPresented: $presentErrorDetails) {
	if let error = viewModel.error {
	ErrorDetailsView(error: error)
	}
	}
	.sheet(isPresented: $presentErrorDetails, onDismiss: { viewModel.error = nil }) {
	if let error = viewModel.error {
	ErrorDetailsView(error: error)
	}
	}

[gemini-code-assist]

In SwiftUI, TextEditor has a default background color that covers any custom background modifier. To ensure that the .secondarySystemGroupedBackground color is visible behind the text editor, you should hide the default scroll content background using .scrollContentBackground(.hidden).

        TextEditor(text: $viewModel.inputText)
          .frame(height: 120)
          .scrollContentBackground(.hidden)
          .padding(4)
          .background(Color(.secondarySystemGroupedBackground))
          .cornerRadius(8)