Enhance REST tests for bucket_sort (#91221)

1. Moves integration tests from an `ESIntegTestCase` into a REST test so
   we can do forwards compatibility tests with it later.
2. Moves the `bucket_sort` pipeline agg from `server` into
   `modules:aggregations` so it'll be a little easier to test it.

Author: nik9000

modules/aggregations/src/main/java/org/elasticsearch/aggregations/AggregationsPlugin.java

@@ -12,6 +12,7 @@
 import org.elasticsearch.aggregations.bucket.adjacency.InternalAdjacencyMatrix;
 import org.elasticsearch.aggregations.bucket.histogram.AutoDateHistogramAggregationBuilder;
 import org.elasticsearch.aggregations.bucket.histogram.InternalAutoDateHistogram;
+import org.elasticsearch.aggregations.pipeline.BucketSortPipelineAggregationBuilder;
 import org.elasticsearch.aggregations.pipeline.Derivative;
 import org.elasticsearch.aggregations.pipeline.DerivativePipelineAggregationBuilder;
 import org.elasticsearch.aggregations.pipeline.MovFnPipelineAggregationBuilder;
@@ -44,6 +45,11 @@ public List<AggregationSpec> getAggregations() {
     @Override
     public List<PipelineAggregationSpec> getPipelineAggregations() {
         return List.of(
+            new PipelineAggregationSpec(
+                BucketSortPipelineAggregationBuilder.NAME,
+                BucketSortPipelineAggregationBuilder::new,
+                BucketSortPipelineAggregationBuilder::parse
+            ),
             new PipelineAggregationSpec(
                 DerivativePipelineAggregationBuilder.NAME,
                 DerivativePipelineAggregationBuilder::new,

server/src/main/java/org/elasticsearch/search/aggregations/pipeline/BucketSortPipelineAggregationBuilder.java renamed to modules/aggregations/src/main/java/org/elasticsearch/aggregations/pipeline/BucketSortPipelineAggregationBuilder.java

@@ -5,12 +5,14 @@
  * in compliance with, at your election, the Elastic License 2.0 or the Server
  * Side Public License, v 1.
  */
-package org.elasticsearch.search.aggregations.pipeline;
+package org.elasticsearch.aggregations.pipeline;
 
 import org.elasticsearch.Version;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.search.aggregations.pipeline.AbstractPipelineAggregationBuilder;
 import org.elasticsearch.search.aggregations.pipeline.BucketHelpers.GapPolicy;
+import org.elasticsearch.search.aggregations.pipeline.PipelineAggregator;
 import org.elasticsearch.search.builder.SearchSourceBuilder;
 import org.elasticsearch.search.sort.FieldSortBuilder;
 import org.elasticsearch.search.sort.SortBuilder;

server/src/main/java/org/elasticsearch/search/aggregations/pipeline/BucketSortPipelineAggregator.java renamed to modules/aggregations/src/main/java/org/elasticsearch/aggregations/pipeline/BucketSortPipelineAggregator.java

@@ -5,13 +5,15 @@
  * in compliance with, at your election, the Elastic License 2.0 or the Server
  * Side Public License, v 1.
  */
-package org.elasticsearch.search.aggregations.pipeline;
+package org.elasticsearch.aggregations.pipeline;
 
 import org.elasticsearch.search.aggregations.AggregationReduceContext;
 import org.elasticsearch.search.aggregations.InternalAggregation;
 import org.elasticsearch.search.aggregations.InternalMultiBucketAggregation;
 import org.elasticsearch.search.aggregations.bucket.MultiBucketsAggregation;
+import org.elasticsearch.search.aggregations.pipeline.BucketHelpers;
 import org.elasticsearch.search.aggregations.pipeline.BucketHelpers.GapPolicy;
+import org.elasticsearch.search.aggregations.pipeline.PipelineAggregator;
 import org.elasticsearch.search.sort.FieldSortBuilder;
 import org.elasticsearch.search.sort.SortOrder;
 

server/src/test/java/org/elasticsearch/search/aggregations/pipeline/BucketSortTests.java renamed to modules/aggregations/src/test/java/org/elasticsearch/aggregations/pipeline/BucketSortPipelineAggregationBuilderTests.java

@@ -5,9 +5,12 @@
  * in compliance with, at your election, the Elastic License 2.0 or the Server
  * Side Public License, v 1.
  */
-package org.elasticsearch.search.aggregations.pipeline;
+package org.elasticsearch.aggregations.pipeline;
 
+import org.elasticsearch.aggregations.AggregationsPlugin;
+import org.elasticsearch.plugins.SearchPlugin;
 import org.elasticsearch.search.aggregations.BasePipelineAggregationTestCase;
+import org.elasticsearch.search.aggregations.pipeline.BucketHelpers;
 import org.elasticsearch.search.sort.FieldSortBuilder;
 import org.elasticsearch.search.sort.SortOrder;
 
@@ -19,7 +22,11 @@
 import static java.util.Collections.singletonList;
 import static org.hamcrest.Matchers.equalTo;
 
-public class BucketSortTests extends BasePipelineAggregationTestCase<BucketSortPipelineAggregationBuilder> {
+public class BucketSortPipelineAggregationBuilderTests extends BasePipelineAggregationTestCase<BucketSortPipelineAggregationBuilder> {
+    @Override
+    protected List<SearchPlugin> plugins() {
+        return List.of(new AggregationsPlugin());
+    }
 
     @Override
     protected BucketSortPipelineAggregationBuilder createTestAggregatorFactory() {