Validate the NOT NULL constraint on project_id in push_rules for self-managed

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Summary

Validate the NOT NULL constraint (check_1d23f0a102) on push_rules.project_id for self-managed instances. The GitLab.com validation was done in #607950 (closed) (Task 7). This is the self-managed equivalent, gated to run only on self-managed (return if Gitlab.com_except_jh?), modeled on ValidateRedirectRoutesShardingKeyOnSelfManaged (db/post_migrate/20251210131249).

Parent tracking issue: #499156 (deferred Task 3).

Background

• Constraint added NOT VALID: #605242 (closed) (MR !245553 (merged)).
• .com validation: #607950 (closed) (Task 7).
• Requires NULL project_id rows removed on self-managed first — depends on the self-managed BBM finalize #607954 (deferred Task 2).
• Must run after required stop 19.5.

Related issues

• Parent: #499156 (deferred Task 3)
• .com validation: #607950 (closed)
• Constraint added by: #605242 (closed)
• Blocked by: self-managed BBM finalize #607954 (deferred Task 2)