Issue #3588057: Vary local task plugin definition cache by active domain

Frank Mablyrequested to merge
issue/domain-3588057:3588057-local-task-cache-domain into 3.x

Decorate plugin.manager.menu.local_task so the discovery cache key includes the active domain id.

See https://www.drupal.org/project/domain/issues/3588057 for the diagnosis and reproducer.

Verified live: on a multi-domain instance with system.theme registered as overridable, the active first tab on /admin/structure/block now reflects each domain's own default theme.

Per-request overhead is one extra string concatenation; cache fragmentation is bounded by the number of domains actually visited. The local_task cache tag still invalidates everything together.

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