Cleanup lazy-generation for Ci::Runner#uuid

🤖 Posted by AI Agent on behalf of Tomasz

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What does this MR do and why?

Cleanup lazy-generation for Ci::Runner#uuid

Follow-up for Add UUID for ci_runners entities (!235296 - merged) and Finalize BBM BackfillCiRunnersUuid (!251000 - merged).

It removes the temporary lazy-generation that was added to ensure each Ci::Runner has an uuid, even if the BBM backfill didn't reach a record yet.

The backfill is finalized since 19.4 and the 19.5 required stop has passed, so every runner has a uuid. Instead of the lazy getter, the invariant is now enforced by the databases:

• PostgreSQL: db/post_migrate/20261028125150_add_not_null_constraint_to_ci_runners_uuid.rb adds a NOT NULL check constraint on ci_runners.uuid (propagated to all partitions). The table is table_size: small, so the constraint is validated in the same migration.
• ClickHouse: db/click_house/post_migrate/main/20261028130500_make_siphon_ci_runners_uuid_non_nullable.rb changes siphon_ci_runners.uuid from Nullable(UUID) to UUID, as requested in !235296 (comment 3515462584). Stale NULL rows left from before the backfill are set to the zero UUID first: ClickHouse < 25.12 fails the type change on NULLs, and >= 25.12 requires an explicit DEFAULT. Tested on 25.1.8.25 and 26.2.13.2 (the versions used in CI).

References

• Add UUID for ci_runners entities (!235296 - merged)
• Finalize BBM BackfillCiRunnersUuid (!251000 - merged)
• !235296 (comment 3515462584)

Rollback

• PostgreSQL: down removes the check constraint. No data is changed.
• ClickHouse: down restores uuid Nullable(UUID) without a default. Zero UUIDs written by up are not reverted to NULL: they carry the same "no uuid" meaning, and reverting them would need another data-rewriting mutation. This is noted in the migration.
• Application code: reverting this MR brings back the lazy getter, which is safe with either database state.

Screenshots or screen recordings

N/A - no UI changes.

How to set up and validate locally

1. Run the PostgreSQL migration and check the constraint:

   bin/rails db:migrate

   SELECT conrelid::regclass, conname FROM pg_constraint WHERE conname = 'check_55f3874e19';

2. With ClickHouse configured, run the ClickHouse migrations and check the column:

   bundle exec rake gitlab:clickhouse:migrate

   SHOW CREATE TABLE siphon_ci_runners;

MR acceptance checklist

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