Reject unsupported legacy creative formats per item without losing valid siblings #1839

When a legacy sync_creatives batch contains one unprojectable format_id, the dispatcher raises request-level INVALID_REQUEST before the seller sees any creative. A batch containing two supported display creatives and an unsupported native creative therefore rejects all three.

Unsupported individual formats should produce creative-level action: failed results while supported siblings proceed. Preserve request identity, ordering, dry-run and idempotency behavior. Partial admission must also protect the full submitted creative ID set during delete_missing, refuse assignments against failed submissions, and retain rejection rows in SDK-managed asynchronous completion results.

The generic handler API currently delegates reconciliation and external asynchronous completion to adopters, so any new partial-admission contract must fail safely before mutation when the handler cannot preserve those semantics.

Reported by an SDK integrator; implementation is being handled by the designated agent (no-triage).