Add NOT NULL constraint for project_id in x509 tables

What does this MR do and why?

Contributes to #553481

After migration is finalized - !207068 (merged), all records in x509_issuers and x509_certificates tables should have project_id set.

Changes:

• Add NOT NULL constraint for project_id columns
• Add Rails validation to ensure project_id is present

References

Sharding Key Implementation for `x509_issuers` ... (#553481)

MR acceptance checklist

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Related to #553481