Validate partitioned FK on vulnerability_occurrences

What does this MR do and why?

Completes step 4 of the list partitioning guide for the vulnerability_occurrences → vulnerability_identifiers reference:

1. Synchronously validates the composite foreign key fk_rails_c8661a61eb_p (partition_id, primary_identifier_id) → vulnerability_identifiers(partition_id, id) (added as NOT VALID in !244081 (merged), scheduled for async validation in !247860 (merged)).
2. Removes the old single-column foreign key fk_rails_c8661a61eb on primary_identifier_id.

This matches the pattern used for vulnerability_occurrence_identifiers in !240976 (merged) and is the final remaining step of #596858 (closed).

Dependencies:

• Depends on !244081 (merged) (adds the FK) and !247860 (merged) (schedules async validation) — targets the async-validation branch for a clean diff; will be rebased and retargeted to master once they merge.
• Must only be merged after the async validation of fk_rails_c8661a61eb_p has completed on GitLab.com, so the synchronous validate_foreign_key is a fast metadata-only operation.

Migration output

Up:

main: == 20260730144814 ValidateCompositeFkOnVulnerabilityOccurrences: migrating ====
main: == 20260730144814 ValidateCompositeFkOnVulnerabilityOccurrences: migrated (0.0382s)
main: == 20260730144844 RemoveOldFkOnVulnerabilityOccurrences: migrating ============
main: == 20260730144844 RemoveOldFkOnVulnerabilityOccurrences: migrated (0.0440s) ===

SQL:

ALTER TABLE vulnerability_occurrences VALIDATE CONSTRAINT fk_rails_c8661a61eb_p;

ALTER TABLE vulnerability_occurrences DROP CONSTRAINT IF EXISTS fk_rails_c8661a61eb;

Down: validation is kept (no-op); the old FK is restored with add_concurrent_foreign_key.

db/structure.sql was regenerated with scripts/regenerate-schema.

References

• Related to #596858 (closed)

How to set up and validate locally

1. Run the migrations and inspect the constraints:

   bundle exec rails db:migrate
   psql -d gitlabhq_development_sec -c '\d vulnerability_occurrences' | grep fk_rails_c8661a61eb

   Only fk_rails_c8661a61eb_p should remain, without NOT VALID.

MR acceptance checklist

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