Adopt Foundation Models OS 27 surfaces: image attachments, capabilities, context size

[james-333i]

This updates the Foundation Models adapter for OS 27 while keeping the package building unchanged on Xcode 26.

Image input: Transcript image segments were previously dropped when converting to a FoundationModels transcript. On OS 27 they now map to Transcript.Segment.attachment with an ImageAttachment, built directly for URL sources and via ImageIO decoding for data sources. Verified end to end on an iPhone running iOS 27 (image prompt answered by the on-device model).

Capabilities and context: SystemLanguageModel now exposes contextSize (back-deployed by Apple to OS 26) and supportsImageInput, which reports the OS 27 vision capability and returns false on earlier OS versions.

Generation options: the adapter now sets maximumResponseTokens on FoundationModels.GenerationOptions. The property has existed since OS 26 but was never assigned, so response token limits were silently ignored on the Foundation Models backend. Verified on device: a capped request now terminates at the limit.

The OS 27 references are behind #if compiler(>=6.4) with runtime availability checks, so Xcode 26 builds still compile and the deployment floor is unchanged

[mattt]

Hi @james-333i. Thank you for this. The maximumResponseTokens fix alone is worth having, and the image-attachment mapping is a nice preview of what's coming.

Here's what I'm thinking for the project overall (details in #210):

• Cut a 1.0 soon against the current (OS 26) Foundation Models API, with some cleanup of places where we've grown beyond it.
• Start a 2.0 for the OS 27 API, which I expect to be source-breaking in a few places (session construction, image inputs, generation options).

Since everything here is availability-gated and doesn't change the OS 26 surface, I'm inclined to take it now rather than hold it for 2.0. Two small things before I merge: can you split out the maximumResponseTokens fix as its own commit so it's easy to cherry-pick, and let me look at how contextSize / supportsImageInput line up with what we do for the LanguageModel protocol more broadly?

Does that sound reasonable?

[james-333i]

@mattt Sounds good. I've split the maximumResponseTokens fix into its own commit so it cherry-picks cleanly. On contextSize and supportsImageInput, I kept both as passthroughs on SystemLanguageModel since the LanguageModel protocol has no capability surface today.