Add UNIQUE index with sharding column to ci_runner_machines

What does this MR do and why?

This MR adds a new UNIQUE index on ci_runner_machines that includes the sharding column (organization_id) to support the Cells architecture.

Changes:

• Adds a new unique partitioned index idx_ci_runner_machines_on_runner_id_type_system_xid_org_id on columns: runner_id, runner_type, system_xid, organization_id
• Updates the RunnerManager.for_runner scope to include organization_id in the WHERE clause to leverage the new index
• Creates partition-specific indexes for all three runner types (instance, group, project)

This index will replace the current index_ci_runner_machines_on_runner_id_and_type_and_system_xid index by including the sharding column, which is required for proper query routing in a Cells environment.

Changelog: added

References

Cells: Handle unique index gitlab_ci.group_type... (#562046 - closed)

Screenshots or screen recordings

N/A - Database migration

Before	After
Index without organization_id	Index with organization_id as first column

How to set up and validate locally

1. Run the migration: bundle exec rails db:migrate

2. Verify the new index exists:

   \d ci_runner_machines

3. Check that queries using Ci::RunnerManager.for_runner use the new index in the query plan

   gitlabhq_test_ci=# explain SELECT "ci_runner_machines".*
     FROM "ci_runner_machines"
     WHERE "ci_runner_machines"."organization_id" = 1
       AND "ci_runner_machines"."runner_id" = 1434
       AND "ci_runner_machines"."runner_type" = 3
     LIMIT 11;
   ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
   │                                                                                    QUERY PLAN                                                                                    │
   ├──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
   │ Limit  (cost=0.14..2.17 rows=1 width=342)                                                                                                                                        │
   │   ->  Index Scan using idx_proj_ci_runner_machines_on_runner_id_type_system_xid_org_id on project_type_ci_runner_machines ci_runner_machines  (cost=0.14..2.17 rows=1 width=342) │
   │         Index Cond: ((runner_id = 1434) AND (runner_type = 3) AND (organization_id = 1))                                                                                         │
   └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
   (3 rows)

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