Issue #3582706: Scope unique constraint validation per domain

Frank Mablyrequested to merge
issue/domain_path-3582706:3582706-path-aliases-unable into 3.x

DomainPathUniqueConstraintValidator was using the full DomainPathAliasManager which falls back to core aliasing. This caused aliases from other domains to trigger false duplicate violations.

Fix: use DomainAliasRepository directly to check only aliases for the specific domain.

Also adds two kernel tests:

  • Same alias on different domains should pass validation.
  • Same alias on the same domain should still fail.

Closes #3582706

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