Fix running tests on macOS 15

[atdrendel]

Before making these changes, the tests would crash when targeting my macOS 15 computer in Xcode 26.2.

After the changes, the tests are correctly skipped and the test suite passes.

[Copilot]

Pull request overview

This PR adjusts the availability checks for SystemLanguageModel-based tests so that on macOS 15 / Xcode 26.2 the tests are correctly skipped instead of crashing.

Changes:

• Introduced file-scoped helper booleans that check SystemLanguageModel.default.isAvailable only within an if #available(macOS 26.0, *) guard.
• Updated the @Test annotations in the affected test files to use these helper booleans for their .enabled(if:) conditions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Tests/AnyLanguageModelTests/APICompatibilityFoundationModelsTests.swift	Adds a guarded isFoundationModelsSystemLanguageModelAvailable helper and uses it to conditionally enable the FoundationModels drop-in compatibility test.
Tests/AnyLanguageModelTests/APICompatibilityAnyLanguageModelTests.swift	Adds a guarded isSystemLanguageModelAvailable helper and uses it to conditionally enable the AnyLanguageModel drop-in compatibility test.

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[mattt]

Perfect. Thank you, @atdrendel!