PROD-2309: skip content items whose target container mapping is stale

[5PK]

Summary

• Follow-up to PROD-2309: guard against unresolved content references before push #188. Live retest on 1.0.0-beta.13.22 (ticket comment, 2026-07-21) found 10 content items still hitting the raw server NullReferenceException after that fix — including the ticket's original repro (eurojackpothomelinks) and the loyaltyconfig nested cluster.
• Root cause, confirmed against the target instance's batch/error tables (Batch.AdditionalData payloads + BatchItem.ErrorMessage stack traces for sync 1c55a7fd-us2 → be79aa3a-us2, 2026-07-21): each of the 10 items' own container was deleted/soft-deleted on the target since the last successful mapping. The mapping row survives, but ContainerMapper.getMappedEntity(..., "target") returns null (its local cache file is gone). Nothing checked for that null, and the payload builder fell back to the item's source reference name — which resolves to nothing on the target — so the server's ContentItem constructor dereferences a null content view and throws the same opaque NRE.
• This is a different failure mode than PROD-2309: guard against unresolved content references before push #188's unresolved-content-reference guard (still working correctly — 3-4 items are cleanly skipped for that reason in the same run). Field shape doesn't predict this failure: ~15 items with the identical reference-field shape succeeded, and 3 of the 10 failing items carry no content-reference field at all — the item's own container is the only thing that matters.
• Fix: when the container mapping exists but no longer resolves to a live target container, skip the item before it's shipped, with a message that clearly names the container (not a content reference) as the cause and suggests recreating the container or clearing its mapping.

Test plan

• Added content-batch-processor.test.ts coverage exercising the real ModelMapper/ContainerMapper file-based lookups (not mocked): one case with the mapping row present but no target container file (stale — asserts the item is skipped with the new message, zero payloads), one regression case with a live target container (asserts the item still ships normally).
• npx tsc --noEmit clean.
• Full pusher suite green: 29 suites / 479 tests.