Implement guided generation for GeminiLanguageModel

[mattt]

Related to #27

[Copilot]

Pull request overview

This pull request implements guided generation support (using @Generable and @Guide macros) for GeminiLanguageModel, addressing issue #27. Previously, the model only supported generating String content and would throw a fatal error when attempting to generate structured types. This change enables GeminiLanguageModel to generate structured data using JSON schemas, bringing it to feature parity with other language models like OpenAI and Anthropic.

Changes:

• Removed the String-only restriction and added full support for generating structured Generable types
• Implemented automatic JSON schema generation and response parsing for non-String types
• Added comprehensive test suite following established testing patterns for structured output

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
Sources/AnyLanguageModel/Models/GeminiLanguageModel.swift	Core implementation: removed String-only guards, added schema conversion, JSON response handling for both streaming and non-streaming generation, and helper functions for empty content handling
Tests/AnyLanguageModelTests/GeminiLanguageModelTests.swift	Added structured output test suite with tests for basic generation, optional fields, different types, and streaming

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