Add partial identity join sorting test

[jechol]

Summary

Adds a regression test for sorting a many_to_many relationship by a join resource attribute when the join resource uses a partial identity.

This covers the behavior fixed in ash-project/ash#2746.

The test uses a polymorphic join resource shape where multiple nullable foreign keys can exist on the same resource, but only the post_id / user_id pair is unique when both keys are present:

identity :post_user, [:post_id, :user_id],
  where: expr(not is_nil(post_id) and not is_nil(user_id))

The loaded relationship then sorts through the join relationship field:

sort: [Ash.Sort.expr_sort(parent(user_notification_recipients.order), :integer)]

Notes

This test fixture mirrors polymorphic ordered join resources where a partial identity guarantees one join row for a specific source/destination pair.

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