Add NOT NULL for sharding key on work_item_parent_links

What does this MR do and why?

Add NOT NULL constraint to work_item_parent_links.namespace_id column.

• We do not add a validation on the model because we're populating the column using a "BEFORE INSERT OR UPDATE" trigger.
• We have finalized the backfill migration for this column.

The sharding key will be used to determine what Organization the row belongs to.

References

• Organization Isolation and Sharding Keys: https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/organization/isolation/
• Sharding Key (Cells) Development Guide: https://docs.gitlab.com/ee/development/cells/#defining-a-sharding-key-for-all-cell-local-tables
• Related to #493768

MR acceptance checklist

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Screenshots or screen recordings

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How to set up and validate locally

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db/post_migrate/20250124055844_add_work_item_parent_links_namespace_id_not_null_constraint.rb

+# frozen_string_literal: true
+
+class AddWorkItemParentLinksNamespaceIdNotNullConstraint < Gitlab::Database::Migration[2.2]
+  disable_ddl_transaction!
+  milestone '17.9'
+
+  def up
+    add_not_null_constraint :work_item_parent_links, :namespace_id, validate: false
+  end
+
+  def down
+    remove_not_null_constraint :work_item_parent_links, :namespace_id
+  end
+end