Add GitLab Shell topology service cell_endpoint config

What does this MR do?

Contributes to gitlab-org/gitlab-shell#860 (closed)

Problem

GitLab Shell MR 1505 makes topology_service.cell_endpoint (scheme and port) required whenever the Topology Service is enabled, and always overrides any port returned by the Topology Service. The chart renders the topology_service block but has no way to set cell_endpoint, so GitLab Shell cannot start once it picks up the new required field.

Solution

• Add config.topologyService.cellEndpoint.scheme and .port values, rendered into the GitLab Shell config.yml topology_service block.
• Guard both fields with required so rendering fails fast when config.topologyService.enabled is true but they are unset, matching the upstream contract.
• Constrain port to 1-65535 and scheme to http/https in values.schema.json.
• Document the new fields and add spec coverage.

This is not a breaking change because the new values have sensible defaults.

Related issues

gitlab-org/gitlab-shell#860 (closed)

Author checklist

For general guidance, please follow our Contributing guide.

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• 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?
  • Describe how you did it in a comment.
• Equivalent MR/issue for omnibus-gitlab opened. - agreed to postpone, since the feature is GitLab.com only

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.