Add NOT NULL constraint

What does this MR do and why?

This is a post deploy migration to add multi column NOT NULL to snippet_repositories as described in #550705 (closed)

• Here is the MR adding model validations and code support: !205897 (merged)
• Here is a comment with links showing backfill migrations: !205897 (comment 2781397210)

migration output

   be rake db:migrate
main: == [advisory_lock_connection] object_id: 155740, pg_backend_pid: 84965
main: == 20250926194922 AddMultiColumnNotNullConstraintToSnippetRepositories: migrating
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- execute("ALTER TABLE snippet_repositories VALIDATE CONSTRAINT check_0d1a38de4c;")
main:    -> 0.0011s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20250926194922 AddMultiColumnNotNullConstraintToSnippetRepositories: migrated (0.0396s)

main: == [advisory_lock_connection] object_id: 155740, pg_backend_pid: 84965
ci: == [advisory_lock_connection] object_id: 155740, pg_backend_pid: 84967
ci: == 20250926194922 AddMultiColumnNotNullConstraintToSnippetRepositories: migrating
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- execute("ALTER TABLE snippet_repositories VALIDATE CONSTRAINT check_0d1a38de4c;")
ci:    -> 0.0016s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20250926194922 AddMultiColumnNotNullConstraintToSnippetRepositories: migrated (0.0162s)

ci: == [advisory_lock_connection] object_id: 155740, pg_backend_pid: 84967

Related to #550705 (closed)