Implement guided generation for CoreML

[mattt]

Related to #27

[mattt]

Currently one failing test:

❌ Test structuredGenerationSimpleInt() recorded an issue at CoreMLLanguageModelTests.swift:194:10: Caught error: numberOutOfRange("Invalid integer: <unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk><unk>")

[Copilot]

Pull request overview

This PR adds guided/structured JSON generation support for the CoreML backend by wiring CoreML into the existing ConstrainedJSONGenerator infrastructure (token-level constrained sampling), and updates structured generation to be async-compatible across backends.

Changes:

• Made TokenBackend.decode/sample and ConstrainedJSONGenerator.generate() async, updating all call sites and tests accordingly.
• Implemented CoreML structured generation via a new CoreML token backend that applies logits processors and masks logits to the schema-allowed token set.
• Added CoreML structured generation tests and adjusted CoreML test suite execution to run serially.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Tests/AnyLanguageModelTests/StructuredGenerationTests.swift	Updates structured generation tests to use async generate().
Tests/AnyLanguageModelTests/Shared/MockTokenBackend.swift	Updates mock backend to conform to async TokenBackend API.
Tests/AnyLanguageModelTests/CoreMLLanguageModelTests.swift	Serializes CoreML suite, caches model download/compile in a task, and adds structured generation tests.
Sources/AnyLanguageModel/StructuredGeneration.swift	Converts constrained generation to async and skips special tokens for numeric token sets.
Sources/AnyLanguageModel/Models/MLXLanguageModel.swift	Adapts structured generation path to async generate(), updates backend methods.
Sources/AnyLanguageModel/Models/LlamaLanguageModel.swift	Adapts structured generation to async generate() and updates backend methods (currently broken due to pointer lifetime issue).
Sources/AnyLanguageModel/Models/CoreMLLanguageModel.swift	Adds structured generation for CoreML (schema prompting + constrained sampling backend) and adjusts string generation output handling.
Package.resolved	Adds/updates Swift package dependencies used by the CoreML implementation.

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