Enable detaching archived partitions of job_definitions & job_definition_instances tables

What does this MR do and why?

Context

We want to reclaim disk by detaching, then dropping, the archived partitions of p_ci_job_definitions and p_ci_job_definition_instances (#552078).

In earlier MRs on that issue we merged the shared machinery this depends on: DetachEligibility refuses a detach that Postgres would reject outright or that would force a scan of a referencing table, and PartitionManager supports detaching concurrently as a per-table opt-in. Neither table is set up to detach anything yet. This MR does that.

This MR

• Ci::Partitionable.partitionable gains a detach_archived: option, default false, turned on only for Ci::JobDefinition and Ci::JobDefinitionInstance; no other CI partitionable model changes behavior.
• Ci::Partition gains a predicate for whether every value in a set is archived, and CiSlidingListStrategy#extra_partitions returns the oldest contiguous run of partitions that predicate accepts, never the most recent one.
• We set detach_concurrently: true for all CI partitionable models. It stays inert for a model that never detaches, since PartitionManager only reads it once there are partitions to detach.
• The whole path sits behind ci_detach_archived_partitions (gitlab_com_derisk, default off, :instance actor), so nothing detaches until it's enabled.

Feature Flag: ci_detach_archived_partitions

References

• Parent issue: Detach and drop archived partitions of p_ci_job... (#552078)
• Task: Wire archived CI partitions through `detach_arc... (#617930 - closed)
• Feature flag roll out: Feature flag rollout: Enable `ci_detach_archive... (#617932)

Database queries

See MR comments.

How to set up and validate locally

1. Run the specs:

   bundle exec rspec spec/models/concerns/ci/partitionable_spec.rb \
     spec/models/ci/partition_spec.rb \
     spec/lib/gitlab/database/partitioning/ci_sliding_list_strategy_spec.rb

2. In a Rails console, archive a partition that is not the current one and check the predicate with the flag on, then off:

   Feature.enable(:ci_detach_archived_partitions)
   
   strategy = Ci::JobDefinition.partitioning_strategy
   database_partition = Gitlab::Database::Partitioning::MultipleNumericListPartition
     .new(:p_ci_job_definitions, [100])
   
   Ci::Partition.find(100).update!(status: Ci::Partition.statuses[:archived])
   strategy.detach_partition_if.call(database_partition)   # => true
   
   Feature.disable(:ci_detach_archived_partitions)
   strategy.detach_partition_if.call(database_partition)   # => false
   
   Ci::Partition.find(100).update!(status: Ci::Partition.statuses[:active])

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