Drop sharding_key_id check constraint on runner models

What does this MR do and why?

This MR:

• drops the sharding_key_id check constraint on runner models.
• stops updating sharding_key_id values for existing models
• ensures that new Ci::RunnerManager models are populated with a valid value (0) if the parent runner has a NULL sharding_key_id. This should avoid a repeat of gitlab-com/gl-infra/production#20245 (closed) (details).

Changelog: other

References

• Ignore sharding_key_id column in runner models (#547654 - closed)
• Ignore `sharding_key_id` column on runner models (!197682 - merged)
• Revert "Merge branch 'pedropombeiro/547654/igno... (!198810 - merged)
• 2025-07-22: QA smoke tests failing due to pipel... (gitlab-com/gl-infra/production#20245 - closed)

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