Fix collapse interaction with stored fields (elastic#112761)

Collapse dynamically will add values to the DocumentField values array.
There are a few scenarios where this is immutable and most of these are
OK. However, we get in trouble when we create an immutable set for
StoredValues which collapse later tries to update.

The other option for this fix was to make an array copy for `values` in
every `DocumentField` ctor, this seemed very expensive and could get out
of hand. So, I decided to fix this one bug instead.

closes elastic#112646

docs/changelog/112761.yaml

@@ -0,0 +1,6 @@
+pr: 112761
+summary: Fix collapse interaction with stored fields
+area: Search
+type: bug
+issues:
+ - 112646

server/src/internalClusterTest/java/org/elasticsearch/search/CollapseSearchResultsIT.java

@@ -14,6 +14,7 @@
 import org.elasticsearch.index.query.MatchAllQueryBuilder;
 import org.elasticsearch.search.collapse.CollapseBuilder;
 import org.elasticsearch.test.ESIntegTestCase;
+import org.elasticsearch.xcontent.XContentType;
 
 import java.util.Map;
 import java.util.Set;
@@ -85,4 +86,33 @@ public void testCollapseWithFields() {
             }
         );
     }
+
+    public void testCollapseWithStoredFields() {
+        final String indexName = "test_collapse";
+        createIndex(indexName);
+        final String collapseField = "collapse_field";
+        assertAcked(indicesAdmin().preparePutMapping(indexName).setSource("""
+                     {
+                       "dynamic": "strict",
+                       "properties": {
+                         "collapse_field":  { "type": "keyword", "store": true },
+                         "ts":  { "type": "date", "store": true }
+                       }
+                     }
+            """, XContentType.JSON));
+        index(indexName, "id_1_0", Map.of(collapseField, "value1", "ts", 0));
+        index(indexName, "id_1_1", Map.of(collapseField, "value1", "ts", 1));
+        index(indexName, "id_2_0", Map.of(collapseField, "value2", "ts", 2));
+        refresh(indexName);
+
+        assertNoFailuresAndResponse(
+            prepareSearch(indexName).setQuery(new MatchAllQueryBuilder())
+                .setFetchSource(false)
+                .storedFields("*")
+                .setCollapse(new CollapseBuilder(collapseField)),
+            searchResponse -> {
+                assertEquals(collapseField, searchResponse.getHits().getCollapseField());
+            }
+        );
+    }
 }

server/src/main/java/org/elasticsearch/search/fetch/subphase/StoredFieldsPhase.java

@@ -27,6 +27,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.stream.Collectors;
 
 /**
  * Process stored fields loaded from a HitContext into DocumentFields
@@ -42,7 +43,8 @@ List<Object> process(Map<String, List<Object>> loadedFields) {
             if (inputs == null) {
                 return List.of();
             }
-            return inputs.stream().map(ft::valueForDisplay).toList();
+            // This is eventually provided to DocumentField, which needs this collection to be mutable
+            return inputs.stream().map(ft::valueForDisplay).collect(Collectors.toList());
         }
 
         boolean hasValue(Map<String, List<Object>> loadedFields) {