Migrate duplicate push_rules to project_push_rules

What does this MR do and why?

This MR migrates duplicate push_rules to project_push_rules using environment-specific logic:

• GitLab.com: Selects duplicate with lowest ID (MIN) to match order(:id) behavior when push_rule_ordered_by_id feature flag is enabled
• Self-managed: Uses LIMIT 1 without ORDER BY to preserve implicit database ordering that customers have been relying on

This maintains consistency with existing project.push_rule association behavior across different environments.

Query Plans

• The migrate_duplicates_with_explicit_ordering(sub_batch) uses the order provided in code: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45521/commands/139530
• The migrate_duplicates_with_implicit_ordering(sub_batch) uses 4 internal queries for lookup
  • First to find project_ids that have duplicate push rules: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45521/commands/139531
  • Second to check if any project_push_rules have already been created for the projects: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45521/commands/139532
  • Third query which mimics the current call we make when we call project.push_rule in app: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45521/commands/139533
  • Then a 4th query which is an insert

References

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Before	After

How to set up and validate locally

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Related to #499156