Backport of 'Fix Zoekt indexing by cleaning up replicas without indices'

What does this MR do and why?

This MR backports !222058 (merged) which fixes a critical bug in Zoekt indexing for namespaces with missing indices.

Problem: The original fix (MR !221451 (merged)) successfully selected namespaces with missing indices, but indices were still not being created due to a deeper issue in the rollout flow. PlanningService only checked replica count and returned :unchanged for namespaces with correct replica count but 0 indices, causing ProvisioningService to skip them entirely.

Solution: Instead of trying to fix PlanningService's logic, this takes a simpler approach: delete all replicas without indices upfront. This forces a replica count mismatch, which triggers the existing PlanningService flow to recreate them properly with indices.

Customer Impact: Self-managed instances with Zoekt enabled experienced non-functional code search despite proper configuration. Namespaces would have replicas created but no indices, requiring manual intervention to trigger indexing.

Changes:

• Add without_indices scope to Replica model
• Add cleanup in RolloutService that runs before SelectionService
• Use batched deletion (each_batch) for efficiency and safety
• Add comprehensive test coverage for cleanup functionality

Backport notes for 18.7:

• Cherry-picked cleanly with no conflicts ✅

Related:

• Original MR: !222058 (merged)
• Reverted MR: !221451 (merged)
• Original issue: #588267 (closed)
• Request for Help: https://gitlab.com/gitlab-com/request-for-help/-/issues/4126

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
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• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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