Add custom JSON mode generation option for Gemini language model

[mattt]

Resolves #79

[Copilot]

Pull request overview

This PR adds JSON mode support for Gemini language models, enabling structured JSON output generation. The implementation introduces a new JSONMode enum with three states: disabled, enabled (basic JSON), and schema-constrained JSON output.

Key changes:

• Added a JSONMode enum to CustomGenerationOptions with support for basic JSON mode and schema-based JSON output
• Integrated JSON mode configuration into the Gemini API request generation logic by setting responseMimeType and optionally responseSchema
• Added comprehensive test coverage for both basic JSON mode and schema-constrained JSON mode

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Sources/AnyLanguageModel/Models/GeminiLanguageModel.swift	Implements the JSONMode enum and integrates it into the request generation logic, setting the appropriate Gemini API parameters
Tests/AnyLanguageModelTests/GeminiLanguageModelTests.swift	Adds tests for JSON mode functionality, including basic JSON mode and schema-constrained mode

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