Dry-run preview UI for retention at all three scopes (#3604793)

Frank Mablyrequested to merge
issue/orchestra-3604793:feature/retention-dry-run-ui into 1.x

Phase 3 of #3604793, on the same issue fork. Adds the dry-run preview UI that Phases 1 (site-wide cleanup) and 2 (per-workflow/per-tenant overrides + hold) were missing: a live count, on each retention form, of how many instances the current policy would purge right now, in that form's scope, without deleting anything.

What changed

  • RetentionManager::previewEligible($scope_type, $scope_id) counts eligible instances site-wide, for one workflow, or for one tenant. It reuses the existing eligibility, precedence (workflow override > tenant override > site default) and hold logic by ANDing a single scope filter into every query, so the preview can never disagree with what the purge would actually do.
  • A shared RetentionPreviewTrait renders the same preview line on all three forms: the site Retention settings, the per-workflow Retention tab, and a new per-tenant Retention form.
  • Tenants get an admin surface for the first time: a Tenants list (TenantListBuilder) under the Orchestra admin menu, showing each tenant's effective override and linking to its TenantRetentionForm. The override is still stored on the tenant config entity (Phase 2); the form is only a UI surface, mirroring the workflow form.

Tests

  • InstanceRetentionTest: two new scoped-preview cases (workflow scope incl. a held workflow previewing zero; tenant scope). Full kernel suite green (12 tests).
  • New functional RetentionUiTest: drives all three preview surfaces, the Tenants list and saving a tenant override (8 assertions, green in Docker).
  • phpcs clean; phpstan no new errors.

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