fix(genai): ensure resource cleanup and fix async execution in content-cache tests

[angelcaamal]

Description

This PR fixes several execution and lifecycle issues in the genai/content-cache tests.

Previously, the tests had the following issues:

1. Skipped executions: The describe block was marked as async, which is not supported by Mocha and caused the suite to exit early or register 0 passing tests.
2. Broken retry delays: The retry delay function was placed inside a before hook. Since before only runs once per suite, test retries were not getting the intended backoff delay.
3. Orphaned resources: If a test failed in the middle of the suite (e.g., during update or use), the delete test was never reached, leaving orphaned caches in the GCP project.

Fixes Internal: b/548501421

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• 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 pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow 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

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

[gemini-code-assist]

Code Review

This pull request refactors the content-cache-create-use-update-delete.test.js test suite by removing the async keyword from the describe block, moving the initial delay into a before hook, and adding an after hook to ensure the content cache is cleaned up. Feedback was provided pointing out that using a before hook for the retry delay will only run once and won't execute on test retries. It is recommended to use a beforeEach hook with this.currentTest instead.

[Kef131]

Looks solid to me 👍

[XrossFox]

LGTM