fix: availability in SystemLanguageModel

[bsorrentino]

In the SystemLanguageModel there are

• extension FoundationModels.GenerationSchema
• extension FoundationModels.GeneratedContent
• extension AnyLanguageModel.GeneratedContent
• extension Tool

That are marked only with @available(macOS 26.0, *) and not with @available(macOS 26.0, iOS 26.0, watchOS 26.0, tvOS 26.0, visionOS 26.0, *)

This cause an error on Xcode 26 where the project target is an IOS version < 26.0

[gaelic-ghost]

I stumbled across the repo and was looking into fixing this real quick, but I ended up with a question or two. I noticed missing availability for iPadOS 26.0 throughout and was wondering whether that's intentional? Given iOS avail is consistently included, it seemed odd that iPadOS was missing. Any show stopper bugs on that I missed, or..?
(Edit: Nvm lol, shows how much I use @available... iOS covers both, apparently)

I also saw a lot of explicit availability for tvOS 26.0+ and watchOS 26.0+ being attributed to blocks that extend, include, or inherit from, symbols such as FoundationModels.Tool or FoundationModels.GenerationSchema that are not available on those platforms at all. In fact, the whole Framework is unavailable on those platforms right now.

I did note the compiler directive for the entire file that prevents this from becoming an issue in practice, but it still looked odd to me to have explicit availability listed that contradicts the code itself. So, thoughts on me removing those erroneous availability values while I'm at it..?

[mattt]

This has been fixed by #63