feat(genaisdk): controlled output generation

[Valeriy-Burlaka]

Description

Adds samples for controlled text generation using the new GenAI SDK. Added region tags:

• googlegenaisdk_ctrlgen_with_enum_schema
• googlegenaisdk_ctrlgen_with_resp_schema
• googlegenaisdk_ctrlgen_with_nullable_schema

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• 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)
• 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/controlled_generation/ctrlgen_with_enum_schema.go:18, tag googlegenaisdk_ctrlgen_with_enum_schema
• genai/controlled_generation/ctrlgen_with_nullable_schema.go:18, tag googlegenaisdk_ctrlgen_with_nullable_schema
• genai/controlled_generation/ctrlgen_with_resp_schema.go:18, tag googlegenaisdk_ctrlgen_with_resp_schema

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[telpirion]

Does this PR overlap with #5178 ?

[telpirion]

I think this PR has been abandoned; let's close it. @Valeriy-Burlaka please reopen when you're ready to resume work. Thanks!