Expose whether a partial update was ignored (target doc missing) via vespa-feed-client Result

Is your feature request related to a problem? Please describe. We perform partial update operations through the vespa-feed-client Java library. When the target document does not exist in an index, the operation is quietly ignored. It would be nice to have a way to know whether the operation was applied or ignored. This isn't a blocker for us — more of a quality-of-life improvement.

To be clear about scope: we are not asking for auto-create behavior. Setting create=true would make the partial update create the missing document, which is not what we want — we want the document to remain absent and to learn that the update was skipped.

A workaround we found is to issue a conditional update with a dummy condition "true" and check the HTTP response code (200 vs 412). This works, but it adds per-operation overhead, changes the operation's semantics (full condition evaluation on every update), and is awkward to thread through the feed client. We're curious whether a more appropriate solution would be possible.

We also looked into the X-Vespa-Ignored-Operation HTTP header, but it does not cover our case. The following partial update (using a sample app schema) is ignored when the target index is empty, yet in Vespa 8.697.20 the response contains no such header. The header is also not surfaced by the vespa-feed-client library.

$ curl -i -X PUT -H "Content-Type:application/json" --data '
  {
      "fields": {
          "title": {
              "assign": "test"
          }
      }
  }' \
  http://localhost:8080/document/v1/news/news/docid/testdoc1

HTTP/1.1 200 OK
Date: Tue, 02 Jun 2026 07:22:11 GMT
Vary: Accept-Encoding
Content-Type: application/json; charset=UTF-8
Transfer-Encoding: chunked

{"pathId":"/document/v1/news/news/docid/testdoc1","id":"id:news:news::testdoc1"}

Describe the solution you'd like

1. Provide a clear signal that a partial update was ignored because the target document did not exist — either by extending X-Vespa-Ignored-Operation to cover this case or by introducing a dedicated header.
2. Surface that signal in the vespa-feed-client ai.vespa.feed.client.Result API, since the header information is not currently exposed there. There would be a method returning boolean or an equivalent enum.