feat(genai): Add samples for Content-cache and Text-generation

[cfloress]

Description

Fixes #

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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 5 region tags.

• genai/content_cache/contentcache_list.go:18, tag googlegenaisdk_contentcache_list
• genai/text_generation/textgen_model_optimizer_with_txt.go:18, tag googlegenaisdk_model_optimizer_textgen_with_txt
• genai/text_generation/textgen_thinking_with_txt.go:18, tag googlegenaisdk_thinking_textgen_with_txt
• genai/text_generation/textgen_with_chat_stream.go:18, tag googlegenaisdk_textgen_chat_stream_with_txt
• genai/text_generation/textgen_with_pdf.go:18, tag googlegenaisdk_textgen_with_pdf

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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 significantly enhances the GenAI Go samples by introducing new examples for both content caching and diverse text generation scenarios. It also includes a crucial update to the underlying GenAI SDK and migrates existing samples to a newer model version, ensuring the samples remain current and demonstrate the latest features and best practices.

Highlights

• New Content Cache Samples: Added a new sample (contentcache_list.go) demonstrating how to retrieve details about a specific cached content resource. This new functionality is also integrated into the existing content cache test suite.
• New Text Generation Samples: Introduced several new samples showcasing advanced text generation capabilities, including streaming chat responses (textgen_with_chat_stream.go), generating text from PDF inputs (textgen_with_pdf.go), demonstrating 'thinking' or step-by-step reasoning (textgen_thinking_with_txt.go), and utilizing model optimization for generation (textgen_model_optimizer_with_txt.go). These new samples are also covered by updated test cases.
• Model Version Updates: Migrated numerous existing GenAI samples across various categories (content cache, controlled generation, token counting, and text generation) from using the gemini-2.0-flash-001 model to the newer and potentially more capable gemini-2.5-flash model.
• SDK Dependency Upgrade: Updated the google.golang.org/genai SDK dependency from version v1.6.0 to v1.17.0, bringing in the latest features, bug fixes, and performance improvements from the GenAI Go client library.

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[gemini-code-assist]

Code Review

The pull request updates the GenAI samples to use the gemini-2.5-flash model and adds new content-cache and text-gen samples. The test file was also updated to skip a test that uses a model that is not consistently available in all test environments.

[msampathkumar]

Looks great! Need minors work. Please review shared comments.

[msampathkumar]

LGTM