feat(genai): Add samples for controlled generation class and nested schema

[cfloress]

Description

Fixes #

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Checklist

• I have followed Sample Format Guide
• pom.xml parent set to latest shared-configuration
• Appropriate changes to README are included in PR
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Tests pass: mvn clean verify required
• Lint passes: mvn -P lint checkstyle:check required
• Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• genai/snippets/src/main/java/genai/controlledgeneration/ControlledGenerationWithClassSchema.java:19, tag googlegenaisdk_ctrlgen_with_class_schema
• genai/snippets/src/main/java/genai/controlledgeneration/ControlledGenerationWithNestedSchema.java:19, tag googlegenaisdk_ctrlgen_with_nested_class_schema

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[gemini-code-assist]

Summary of Changes

Hello @cfloress, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request expands the Google GenAI SDK's sample coverage by adding new Java examples for controlled generation. These samples illustrate how to guide the AI model to produce structured output using predefined schemas, including scenarios with simple class structures and more complex nested schemas involving enums. The addition of corresponding integration tests ensures the reliability of these new functionalities.

Highlights

• New Controlled Generation Sample (Class Schema): Added a new Java sample demonstrating how to use a class-based schema for controlled generation with the Google GenAI SDK, specifically for generating structured recipe data.
• New Controlled Generation Sample (Nested Schema): Introduced another Java sample showcasing controlled generation with a nested schema, including an enum type, to generate more complex structured data like recipes with ratings.
• Integration Tests: Included new integration tests to validate the functionality and correctness of both the class-based and nested schema controlled generation samples.

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[gemini-code-assist]

Code Review

This pull request adds two new samples demonstrating controlled generation with class and nested schemas, which are great additions. The code is generally well-written, but I've identified a few areas for improvement. Specifically, there's a potential deserialization bug in the Grade enum that could cause runtime exceptions. I've also suggested some changes to align with standard Java naming conventions for better code maintainability. Please see my detailed comments below.