fix(genai): VideoGenerationIT/ ImageGenerationIT bucket & Cleanup Fixes

[Kef131]

Description

Fixes #
b/549909475

• Allow overriding the default test bucket using the BUCKET_NAME environment variable (System.getenv().getOrDefault("BUCKET_NAME", "java-docs-samples-testing")).
• Wrap the @AfterClass cleanup() method in a try-catch block to prevent build failure during teardown if bucket cleanup fails or permissions are restricted.

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.
• 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

Compliance & Style

• I have followed Sample Format Guide
• pom.xml parent set to latest shared-configuration
• Appropriate changes to README are included in PR
• 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

Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request updates VideoGenerationIT.java to retrieve the bucket name from an environment variable with a fallback default, and wraps the test cleanup logic in a try-catch block to handle exceptions gracefully. The reviewer suggested sanitizing the BUCKET_NAME environment variable to strip any accidental gs:// prefix, which prevents potential Storage SDK failures and makes the tests more robust.