Add APIs for denoting model availability

[mattt]

AnyLanguageModel doesn't currently provide equivalent APIs for language models to communicate that they're unavailable. For example, in the case of the system Foundation Model, Apple Intelligence must be enabled, and that's indicated by Availability.UnavailableReason.appleIntelligenceNotEnabled.

One smell arising from this deficiency is that we needed to @testable import AnyLanguageModel to introspect the underlying Foundation Models API to gate tests.

This PR adds that availability functionality. The LanguageModel protocol adds a new UnavailableReason associated type requirement, which provides some flexibility for implementations to define their own specific reasons. For example, a model that proxies to service providers may report "network unreachable" and a model running locally may report "insufficient RAM".

[Copilot]

Pull Request Overview

This PR adds availability APIs to the LanguageModel protocol, enabling models to communicate whether they're ready to handle requests. This eliminates the need to use @testable import to access underlying model availability states, as seen in the SystemLanguageModel tests.

Key changes:

• Introduces an Availability<UnavailableReason> enum with .available and .unavailable(reason) cases
• Adds UnavailableReason associated type and availability property requirements to the LanguageModel protocol
• Provides default .available implementation for models where UnavailableReason == Never

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Sources/AnyLanguageModel/Availability.swift	New file defining the Availability enum with generic unavailable reason support
Sources/AnyLanguageModel/LanguageModel.swift	Adds availability requirements and convenience property isAvailable to protocol
Sources/AnyLanguageModel/Models/SystemLanguageModel.swift	Implements availability API by bridging to FoundationModels availability, adds static .default accessor
Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift	Declares UnavailableReason = Never for always-available model
Sources/AnyLanguageModel/Models/OllamaLanguageModel.swift	Declares UnavailableReason = Never for always-available model
Sources/AnyLanguageModel/Models/MLXLanguageModel.swift	Declares UnavailableReason = Never for always-available model
Sources/AnyLanguageModel/Models/LlamaLanguageModel.swift	Declares UnavailableReason = Never for always-available model
Sources/AnyLanguageModel/Models/CoreMLLanguageModel.swift	Declares UnavailableReason = Never for always-available model
Sources/AnyLanguageModel/Models/AnthropicLanguageModel.swift	Declares UnavailableReason = Never for always-available model
Tests/AnyLanguageModelTests/SystemLanguageModelTests.swift	Updates test to use new public availability API instead of internal access
Tests/AnyLanguageModelTests/Shared/MockLanguageModel.swift	Implements availability support with customizable provider for testing
Tests/AnyLanguageModelTests/MockLanguageModelTests.swift	Adds test coverage for unavailable model scenarios

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