feat(genai): migrate ImageGenWithText to gemini-2.5-flash-image

[Kef131]

Description

Fixes #
b/549909475

• Migrate ImageGenWithText.java from deprecated Imagen (imagen-4.0-generate-001 via client.models.generateImages) to gemini-2.5-flash-image using client.models.generateContent with responseModalities("TEXT", "IMAGE").
• Update ImageGenerationIT.java to support configurable BUCKET_NAME, add safe error handling in @AfterClass cleanup(), and validate testImageGenWithText against gemini-2.5-flash-image.
• Temporarily mark remaining deprecated Imagen capability tests with @Ignore pending multimodal migration in follow-up PR.

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 migrates the image generation sample and its integration tests from using the deprecated Imagen model to the Gemini model (gemini-2.5-flash-image) via the generateContent API. It also updates the integration tests to handle GCS cleanup errors gracefully and temporarily ignores tests associated with deprecated Imagen models. The reviewer suggested using the %d format specifier instead of %s when printing the image byte length to ensure proper type-safe formatting.