Add bulk partition-aware pipeline lookup and preloader

What does this MR do and why?

Stacked on top of !236791 (merged). ActiveRecord's standard preloader can already load a Ci::Pipeline association for a collection of records in a single WHERE id IN (...) query, but that query has no partition_id filter and therefore scans every partition of p_ci_pipelines. This is the unpruned cross-partition scan that triggered INC-8367 (see #593701 (closed)). This MR adds a custom preloader that uses the pipelines_id_range column on ci_partitions to add partition pruning to the bulk lookup.

Three small layers:

• Ci::Partition.partition_ids_for(pipeline_ids) — single SQL roundtrip that resolves a list of pipeline ids to their tracked partition ids using the pipelines_id_range column. Returns [[pipeline_id, partition_id_or_nil], ...]. Future-proofed against an overrides table via a COALESCE branch.
• Gitlab::Ci::Pipeline::BulkByIdLookup — bulk analogue of ByIdLookup. Applies the same three-step cascade (current partition → range lookup → full scan) across an id array in at most three queries. Each fallback logs the count of unresolved ids.
• Ci::Preloaders::PipelinePreloader — AR-aware preloader that uses BulkByIdLookup and populates the association cache so subsequent reads issue zero queries. Accepts association: and foreign_key: so the same class serves head_pipeline, pipeline, ci_pipeline, latest_pipeline, etc.

The preloader composes with the per-record ByIdLookup override through ActiveRecord's loaded? short-circuit: list endpoints opt in to the preloader for batched loads, single-record reads still go through the per-record path.

No callers are converted yet — subsequent MRs will opt in list endpoints (REST, GraphQL, Sidekiq) one at a time.

References

• Related issue: #599815 (Add partition pruning to pipeline relations not in the CI database)
• Predecessor MR: !236791 (merged) (Use pipelines_id_range to resolve Ci::Pipeline by id)
• Origin issue: #593701 (closed)

Screenshots or screen recordings

N/A — no UI changes.

How to set up and validate locally

In rails console:

mrs = MergeRequest.where.not(head_pipeline_id: nil).limit(20).to_a

Ci::Preloaders::PipelinePreloader.new(mrs, association: :head_pipeline, foreign_key: :head_pipeline_id).preload_all

mrs.each(&:head_pipeline)

Watch the console: preload_all issues at most three queries against p_ci_pipelines, each one filtered by partition_id, and the subsequent head_pipeline reads issue none.

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