feat(genai): Add samples for vertexAI express mode api key

[cfloress]

Description

Mock test for local testing and so avoid api calls
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

[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 introduces new sample code for the Google Cloud Generative AI (GenAI) SDK. The primary purpose of these changes is to demonstrate how to utilize Vertex AI's express mode by authenticating with an API key. This provides developers with a clear example of how to interact with GenAI models using a direct API key authentication method, expanding the available authentication patterns for the SDK.

Highlights

• New Vertex AI Express Mode Sample: A new Go file, expressmode_with_api_key.go, has been added. This file contains the generateContentWithApiKey function, which demonstrates how to initialize the genai.Client with an API key and make a GenerateContent call to a Gemini model, specifically "gemini-2.5-flash".
• Integration Test for API Key Sample: A corresponding test file, expressmode_with_api_key_test.go, has been introduced to validate the functionality of the new sample. This integration test sets up necessary environment variables for Vertex AI and verifies that the generateContentWithApiKey function produces non-empty output, skipping the test if the required API key is not configured.

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[gemini-code-assist]

Code Review

This pull request adds a new Go sample for using Vertex AI Express Mode with an API key. The changes are a good start, but there are a few critical issues in both the sample code and the test that need to be addressed. The main issues are related to Go naming conventions, incorrect handling of the API key which makes the sample and test non-functional, and a missing documentation tag. I've left specific comments with suggestions to fix these problems.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• genai/express_mode/expressmode_with_api_key.go:18, tag googlegenaisdk_vertexai_express_mode

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