Allow independent annotations for toolbox backup pods

What does this MR do?

Adds gitlab.toolbox.backups.cron.annotations so backup pods can add or override metadata without changing annotations on the long-running Toolbox Deployment. Backup pod annotations merge with gitlab.toolbox.annotations; backup-specific values take precedence when both maps contain the same key.

This is useful for logging, monitoring, reloaders, and policy integrations that need backup-specific metadata while preserving shared Toolbox annotations.

Related issues

Closes #6170 (closed)

Test plan

Rendered the backup CronJob with Helm 4.2.3 and confirmed:

• With only gitlab.toolbox.annotations, the backup pod inherits the shared annotations.
• With gitlab.toolbox.backups.cron.annotations, the backup pod retains shared keys, adds backup-only keys, and uses the backup value for overlapping keys.
• The checksum and safe-to-evict annotations remain present.

Added RSpec coverage for the inheritance and merged override paths.

Author checklist

Required

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated.
• Have you validated that your change works end-to-end on a real cluster deployment?
  • I validated both rendered manifests locally. I did not deploy this metadata-only change to a cluster.
• Equivalent MR/issue for omnibus-gitlab opened.
  • Not applicable; this only changes pod metadata in the Helm chart.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.