feat(genai): Add samples for controlled generation

[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 3 region tags.

• genai/snippets/src/main/java/genai/controlledgeneration/ControlledGenerationWithEnumClassSchema.java:19, tag googlegenaisdk_ctrlgen_with_enum_class_schema
• genai/snippets/src/main/java/genai/controlledgeneration/ControlledGenerationWithNullableSchema.java:19, tag googlegenaisdk_ctrlgen_with_nullable_schema
• genai/snippets/src/main/java/genai/controlledgeneration/ControlledGenerationWithResponseSchema.java:19, tag googlegenaisdk_ctrlgen_with_resp_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 enhances the genai samples by adding three new examples for controlled generation using the Google GenAI SDK in Java. These samples illustrate how to leverage different schema types—enum, nullable, and complex object/array schemas—to precisely define the structure and content of the model's output, ensuring more predictable and usable responses.

Highlights

• Enum Class Schema Sample: Introduces a new sample demonstrating how to use an enum class to define a schema, constraining the model's output to a specific set of string values.
• Nullable Schema Sample: Adds a sample showcasing the use of nullable properties within a schema, allowing for optional fields in the model's structured output.
• Response Schema Sample: Provides a sample for defining a complex response schema, including objects and arrays, to guide the model in generating structured data like recipe lists.
• Integration Tests: Includes new integration tests for each of the controlled generation samples to ensure their functionality and correctness.

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[gemini-code-assist]

Code Review

This pull request introduces new samples for controlled generation with different schema types, along with their integration tests. The code is well-structured and the samples are clear. My review focuses on improving maintainability, code clarity, and test robustness. I've suggested dynamically building schema parts, handling return values explicitly, and strengthening test assertions to properly validate the structured output from the model. I also pointed out a minor naming inconsistency for better code uniformity.