A partial document is not an upsert source

Elasticsearch’s Update API supports two source documents with different jobs. doc is merged into a document that already exists. A separate upsert object is inserted when the ID is absent. They need not contain the same fields:

{
  "doc": {"price": 42},
  "upsert": {"title": "Initial", "price": 10}
}

The gateway already supported doc_as_upsert, but it required doc to double as the creation source. It now accepts the separate form for both standalone and bulk updates. An absent ID writes the upsert object and returns HTTP 201 with result: "created"; an existing ID continues to merge doc and ignores the creation source.

The regression test exercises both gateway storage layouts, verifies the exact _source sent to Qdrant, and proves that creation uses one existence lookup rather than the two previously incurred by the doc_as_upsert path:

cargo test supported_update_upserts_write_selected_source_with_one_preflight_each

This is not support for scripted updates. It also does not make the lookup and insert atomic across gateway replicas, implement Elasticsearch sequence numbers, or remove the normal projection-write costs. Invalid non-object upsert values are rejected before any document lookup or write.