Change initializer apiKey parameter to be a token provider closure

[mattt]

This PR updates the initializers for AnthropicLanguageModel and OpenAILanguageModel to accept closures for the apiKey parameter. By using @autoclosure, we can maintain the existing usage providing String arguments (such as ProcessInfo.processInfo.environment["ANTHROPIC_API_KEY"].

Making this a closure allows callers to dynamically inject credentials, such as with an OAuth manager.

[Copilot]

Pull Request Overview

This PR refactors the API key handling in AnthropicLanguageModel and OpenAILanguageModel to use closure-based token providers instead of static strings. By leveraging @autoclosure, existing code passing string literals continues to work unchanged, while new use cases can dynamically provide credentials (e.g., through OAuth managers).

Key Changes:

• Replaced apiKey: String parameters with @autoclosure closures that return String
• Made token providers private and invoked them at request time rather than initialization
• Removed cached headers in AnthropicLanguageModel, rebuilding them per-request to use fresh tokens

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
Sources/AnyLanguageModel/Models/OpenAILanguageModel.swift	Changed apiKey to a closure-based tokenProvider, updated all authorization headers to invoke the provider
Sources/AnyLanguageModel/Models/AnthropicLanguageModel.swift	Changed apiKey to a closure-based tokenProvider, removed cached headers field, and rebuilt headers per-request
Tests/AnyLanguageModelTests/OpenAILanguageModelTests.swift	Removed test assertion for apiKey property since it's now private

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