Implement guided generation for AnthropicLanguageModel

[mattt]

Related to #27

[Copilot]

Pull request overview

This PR implements guided generation support for AnthropicLanguageModel, enabling it to generate structured data types annotated with @Generable instead of just strings. This addresses issue #27, which requested support for the @Generable protocol and @Guide macro.

Changes:

• Removed fatal error guards that prevented non-String content generation in both streaming and non-streaming response methods
• Added schema conversion logic to translate GenerationSchema to Anthropic's JSONSchema format
• Implemented structured response handling with proper JSON parsing for both streaming and non-streaming modes
• Added helper functions for empty content handling, partial snapshot creation, and schema format conversion
• Refactored tool schema conversion to reuse the new schema conversion function

Reviewed changes

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

File	Description
Sources/AnyLanguageModel/Models/AnthropicLanguageModel.swift	Implements guided generation by adding schema-to-JSON conversion, output_config parameter construction, and structured response parsing for both streaming and non-streaming modes
Tests/AnyLanguageModelTests/AnthropicLanguageModelTests.swift	Adds test struct AnthropicStructuredForecast and two new test cases (streamingStructured and structuredResponse) to verify structured generation functionality

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