Create partitioned table p_ci_build_needs

What does this MR do and why?

This MR converts the ci_build_needs table to a list-partitioned table as part of the CI data partitioning initiative. It creates the parent partitioned table p_ci_build_needs and attaches the existing ci_build_needs table as its first partition (covering partition_id values 100–111).

Key changes:

• Model update: Sets sequence_name on Ci::BuildNeed to explicitly use ci_build_needs_id_seq, ensuring correct sequence ownership after the parent table takes over.
• Post-migration: Sync constraint (20260326000459): Validates the existing partitioning_constraint check constraint on ci_build_needs before the conversion.
• Post-migration: Drop old unique index (20260326000462): Removes the legacy index_ci_build_needs_on_build_id_and_name index (non-partition-aware) from ci_build_needs, replaced by the partition-aware index_ci_build_needs_on_build_id_and_name_partitioning.
• Post-migration: Convert to list partition (20260326000464): Calls convert_table_to_first_list_partition to create p_ci_build_needs as the partitioned parent and attach ci_build_needs as the first partition for values 100–111.
• Schema updates: Reflects the new parent table p_ci_build_needs with PARTITION BY LIST (partition_id), updated primary key, indexes, and foreign key constraints.
• Bump sequences fix: Adds .uniq to sequences_by_gitlab_schema to avoid duplicate sequence entries that could arise when both the parent and child tables reference the same sequence.
• Spec/sharding key updates: Registers p_ci_build_needs in the schema spec and sharding key spec, noting that the LFK is already covered via cascade delete from p_ci_builds.
• DB docs: Adds db/docs/p_ci_build_needs.yml documenting the new table with its sharding key, feature category, and schema.

Changelog: added

References

Related to #583639 (closed)

Screenshots or screen recordings

N/A – database migration only

How to set up and validate locally

1. Run the post-migrations in order:

   bundle exec rails db:migrate:up VERSION=20260326000459
   bundle exec rails db:migrate:up VERSION=20260326000462
   bundle exec rails db:migrate:up VERSION=20260326000464

2. Verify the parent table exists and the partition is attached:

   \d+ p_ci_build_needs

3. Confirm ci_build_needs is listed as a partition of p_ci_build_needs for values 100–111.
4. Run rollback to verify down migrations work correctly.

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