Add NOT NULL constraint to push_rules.project_id

What does this MR do and why?

Adds a NOT NULL constraint on push_rules.project_id as NOT VALID (without validating). This is required to make push_rules Cells-compatible with project_id as the sharding key, which must be non-nullable.

Migration output

== 20260715040848 AddNotNullConstraintToPushRulesProjectId: migrating =========
-- execute("ALTER TABLE push_rules\nADD CONSTRAINT check_1d23f0a102\nCHECK ( project_id IS NOT NULL )\nNOT VALID;\n")
   -> 0.0011s
== 20260715040848 AddNotNullConstraintToPushRulesProjectId: migrated (0.0707s)

Related issues

• Closes part of #605242 (closed) (Task 5 of #499156)