A preview call can be quite expensive and cause timeout in a cluster. As a short term fix we identified 2 (hopefully) small fixes:

- make the call cancellable, the main issue here seems to be that preview calls search without using a parent task id(see this fix for data feeds: https://github.com/elastic/elasticsearch/pull/88142/files#diff-aba58244cb8f9dfa3ac3b6efeb02259e3d5956576f56c1ff5e243dd2aac27381)
- pass the timeout parameter of the preview call to the search call, currently a timeout can be specified, but is ignored for the inner search call, which is executed with no timeout (see https://www.elastic.co/guide/en/elasticsearch/reference/master/search-search.html#search-search-api-query-params).

Both changes are bugs and shall be relatively simple to implement and considered for backport to the 7.17 branch.

@przemekwitek can you have a look at this?

related: #88010
x-pack/plugin/transform/src/test/java/org/elasticsearch/xpack/transform/transforms/pivot/PivotTests.java
},
                      "sort": {
                        "field": "timestamp"
                      }
                    }
                  }
                }""");
        }
        if (agg.equals("global")) {
            return parseAggregations("""
                {"pivot_global": {"global": {}}}""");
        }
        return parseAggregations(formatted("""
            {
              "pivot_%s": {
                "%s": {
                  "field": "values"
                }
              }
            }""", agg, agg));
    }
    private AggregationConfig parseAggregations(String json) throws IOException {
        final XContentParser parser = XContentType.JSON.xContent()
            .createParser(XContentParserConfiguration.EMPTY.withRegistry(xContentRegistry()), json);
        
        assertThat(parser.nextToken(), equalTo(XContentParser.Token.START_OBJECT));
        return AggregationConfig.fromXContent(parser, false);
    }
    private static void assertValidTransform(Client client, SourceConfig source, Function pivot) throws Exception {
        validate(client, source, pivot, true);