Use PipelinePreloader for ScheduledPipelineExecutionPolicyTestRun

What does this MR do and why?

Converts the with_associations scope on Security::ScheduledPipelineExecutionPolicyTestRun to use Ci::Preloaders::PipelinePreloader for partition-aware pipeline preloading, instead of relying on ActiveRecord's default preload(:pipeline).

Why

Ci::Pipeline is stored in a partitioned table (p_ci_pipelines). When ActiveRecord's default preloader loads a belongs_to :pipeline association in bulk, it issues a single SELECT ... WHERE id IN (...) query without a partition_id filter. This forces PostgreSQL to scan all partitions, which is inefficient and can cause N+1-style cross-partition queries as the number of partitions grows.

Ci::Preloaders::PipelinePreloader solves this by using Gitlab::Ci::Pipeline::BulkByIdLookup, which resolves the correct partition_id for each pipeline ID (via the cached Ci::Partition ranges) and issues partition-scoped queries. This avoids full partition scans and keeps pipeline lookups efficient as the CI sliding-list partitioning scheme adds new partitions over time.

What

• Extracts a new preload_pipeline scope on ScheduledPipelineExecutionPolicyTestRun that chains with_partition_aware_preload.preload(:pipeline), delegating to the partition-aware preloader.
• Refactors with_associations to call preload_pipeline first, then chains the remaining associations (project: :route, security_policy: :security_policy_management_project) and the order(id: :desc) clause.
• Adds a spec asserting that with_associations preloads the security_policy association (a previously untested association in the scope).

Related: #601100

References

• Issue: #601100
• Ci::Preloaders::PipelinePreloader: app/models/ci/preloaders/pipeline_preloader.rb

Screenshots or screen recordings

Not applicable — backend-only change with no UI impact.

Before	After

How to set up and validate locally

1. Create a project with a security policy that has scheduled pipeline execution policy test runs.
2. Query Security::ScheduledPipelineExecutionPolicyTestRun.with_associations and verify no N+1 queries are issued against p_ci_pipelines across partitions.
3. Run the model spec: bundle exec rspec ee/spec/models/security/scheduled_pipeline_execution_policy_test_run_spec.rb

MR acceptance checklist

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