Include path prefix in Domain::getLink() link label

Frank Mablyrequested to merge
issue/domain-3578566:3578566-domaingetlink-should-include into 3.x

When path prefix mode is enabled, getLink() now appends the prefix to the canonical hostname (e.g. "example.com/fr" instead of just "example.com"). This makes the DomainSwitcherBlock and admin list display distinguishable labels for prefixed domains.

Remove the separate "Prefix" column from DomainListBuilder since the hostname link already includes it. Update the DomainInterface docblock and functional test accordingly.

Closes #3578566

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