Implement guided generation for OllamaLanguageModel

[mattt]

Resolves #27

[Copilot]

Pull request overview

This pull request implements guided generation (structured output) support for the OllamaLanguageModel, allowing it to generate responses conforming to Swift types annotated with @Generable, not just plain strings.

Changes:

• Removed the String-only restriction that previously caused a fatal error for structured types
• Added schema conversion and format parameter support to guide Ollama's output generation
• Updated response parsing logic to handle both String and structured JSON responses in both streaming and non-streaming modes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Sources/AnyLanguageModel/Models/OllamaLanguageModel.swift	Implements structured generation by converting GenerationSchema to JSONSchema format, passing it via the format parameter to Ollama API, and parsing JSON responses into structured types
Tests/AnyLanguageModelTests/OllamaLanguageModelTests.swift	Adds test struct OllamaStructuredForecast and two tests (structuredResponse and streamingStructured) to verify structured generation works correctly

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