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.
• removes validation of sharding_key_id
• ensures that new Ci::RunnerManager models are populated with a valid value (-1) 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

Overall plan

1. Merge MR in %18.3:

   • Drop check constraints (including NOT NULL)
   • Remove app-side validations
   • Keep code that populates the field (for backward compatibility)

2. Merge MR in %18.4:

   • Add ignore_column directives
   • Remove ALL code that references sharding_key_id (both updating existing and populating new)

3. Merge MR in %18.5:

   • Use PDM to physically drop the column

4. Merge MR in %18.6:

   • Remove ignore_column directives

References

• Drop check constraints on sharding_key_id colum... (#557226 - 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)

MR acceptance checklist

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