Fix simulator tests on Xcode 26

[atdrendel]

This pull request builds on my changes in mattt/AnyLanguageModel#100.

• Fix issues building on macOS 15 (Xcode 26.2) targeting the iOS 26.2 Simulator
• Fix issues building for watchOS

Before making these changes, running the tests on my iOS 26.2 Simulator running on macOS 15 (Xcode 26.2) resulted in a bunch of failed tests.

After fixing the availability guards, tests were skipped correctly and the entire test suite passed.

[atdrendel]

Core Image is not available on watchOS.

[atdrendel]

These products are all explicitly imported in GenerableMacro.swift.

[Copilot]

Pull request overview

This PR fixes simulator test failures on iOS 26.2 Simulator when building with Xcode 26.2 on macOS 15 by adding comprehensive platform availability guards. Building on the related PR #100 which fixed macOS 15 tests, this change ensures tests are properly skipped on platforms where SystemLanguageModel features are not available.

Changes:

• Extended availability guards in test files to include iOS, watchOS, tvOS, and visionOS 26.0 alongside existing macOS 26.0 requirements
• Added CoreImage availability checks to MLXLanguageModel import guards for improved watchOS compatibility
• Added missing SwiftSyntax and SwiftSyntaxBuilder dependencies to Package.swift that were already being imported in macro code

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Tests/AnyLanguageModelTests/SystemLanguageModelTests.swift	Extended @available annotations and availability checks to include iOS, watchOS, tvOS, and visionOS 26.0 for all SystemLanguageModel test methods
Tests/AnyLanguageModelTests/APICompatibilityFoundationModelsTests.swift	Added comprehensive platform availability guards (iOS, watchOS, tvOS, visionOS 26.0) to FoundationModels compatibility test
Tests/AnyLanguageModelTests/APICompatibilityAnyLanguageModelTests.swift	Added comprehensive platform availability guards (iOS, watchOS, tvOS, visionOS 26.0) to AnyLanguageModel compatibility test
Sources/AnyLanguageModel/Models/MLXLanguageModel.swift	Added CoreImage availability checks to UIKit and AppKit import guards to ensure CoreImage is available before importing (improves watchOS build compatibility)
Package.swift	Added missing SwiftSyntax and SwiftSyntaxBuilder dependencies that were already being imported in AnyLanguageModelMacros source code

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[mattt]

@atdrendel Thank you for your great work on this and #100 — I really appreciate your help!