Draft: Add NOT NULL constraint for organization_id in scim_oauth_access_tokens

This is part of Add sharding key for `scim_oauth_access_tokens` (#553107) and a follow-up to the following MRs:

• Add `organization_id` to `scim_oauth_access_tok... (!212160 - merged)
• Backfill `organization_id` in `scim_oauth_acces... (!213936 - merged)

What does this MR do and why?

This MR adds a NOT NULL constraint on organization_id for the scim_oauth_access_tokens table to complete the Protocells compliance work.

This is the final MR in the series.

References

• Add sharding key for `scim_oauth_access_tokens` (#553107)
• Add `organization_id` to `scim_oauth_access_tok... (!212160 - merged)
• Backfill `organization_id` in `scim_oauth_acces... (!213936 - merged)

How to set up and validate locally

1. Run the migration:

bundle exec rails db:migrate

2. Verify the constraint exists:

ActiveRecord::Base.connection.execute(<<~SQL).to_a
  SELECT conname, pg_get_constraintdef(oid)
  FROM pg_constraint
  WHERE conrelid = 'scim_oauth_access_tokens'::regclass
  AND contype = 'c'
  AND pg_get_constraintdef(oid) LIKE '%organization_id IS NOT NULL%'
SQL

MR acceptance checklist

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