[Firebase AI] Add Sendable conformance to PartsRepresentable

[andrewheard]

Require Sendable conformance for PartsRepresentable types.

Note: CGImage didn't become Sendable until Swift 6.1 (Xcode 16.3) and NSImage / CIImage became Sendable in Swift 6.2 (Xcode 26.0). As a workaround, these types are marked @unchecked @retroactive Sendable to support building on Xcode 16.0 to 16.2.

#no-changelog

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Code Review

This pull request correctly adds Sendable conformance to the PartsRepresentable protocol and adds the necessary conditional retroactive Sendable conformance for CGImage, NSImage, and CIImage to support older Swift versions. However, the changes introduce compilation errors by removing the public access modifier from the partsValue property in the extensions for these image types. Since PartsRepresentable is a public protocol, its requirements must be implemented with public visibility. My review includes suggestions to fix this.

[andrewheard]

This produces the warning Conformance of 'NSImage' to 'Sendable' is only available in macOS 14.0 or newer on Xcode 26.1 (and potentially other versions of 26).