Add conditional Valkey cache support for Drupal

[jacobaaronyeager]

Provision tagged Cloud.gov ElastiCache infrastructure, bind it when available, and configure Drupal Redis/Predis with TLS while preserving cache fallback.

Jira ticket

DIGITAL-752

Purpose

Install Redis to reduce cache load on Drupal db

Deployment and testing

QA/Testing instructions

(NOTE: AI suggested testing.)

1. Before deployment, record the elapsed time from a comparable dev static-site generation run.

2. Deploy the branch to dev through the normal build-and-deploy workflow. Confirm infrastructure completes before the Drupal deployment.

3. Verify the service and binding without exposing credentials:

sh
cf services
cf env digital-gov-drupal-dev

Confirm digital-gov-cache-dev is bound and its VCAP_SERVICES entry includes the cache-service tag.

4. In the deployed Drupal container, run a cache rebuild and confirm the default cache backend is Redis:

drush cr
drush php:eval 'print \Drupal::service("cache.default")::class . PHP_EOL;'

The class should be in the Drupal\redis namespace.

5. Run static generation again with comparable content and record elapsed time. Compare it with the baseline, along with the RDS size and Drupal instance count.

6. Confirm fallback behavior in a space without digital-gov-cache-: deployment should succeed and Drupal should continue using its normal database cache backend.

Checklist for the Developer

• A link to the JIRA ticket has been included above.
• No merge conflicts exist with the target branch.
• Automated tests have passed on this PR.
• A reviewer has been designated.
• Deployment and testing steps have been documented above, if applicable.

Checklist for the Peer Reviewers

• The file changes are relevant to the task objective.
• Code is readable and includes appropriate commenting.
• Code standards and best practices are followed.
• QA/Test steps were successfully completed, if applicable.
• Applicable logs are free of errors.