Require the name field for inner_hits for collapse (#104666)

`name` is de facto required for `collapse.inner_hits`. It always has been, but we have never validated up front. Instead we accidentally try to serialize `null`, which leads to exciting and confusing errors.

closes: #104647

Author: benwtrent

docs/changelog/104666.yaml

@@ -0,0 +1,5 @@
+pr: 104666
+summary: Require the name field for `inner_hits` for collapse
+area: Search
+type: bug
+issues: []

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search/110_field_collapsing.yml

@@ -515,3 +515,15 @@ setup:
   - match: { hits.hits.2.inner_hits.sub_hits.hits.hits.1._id: "4" }
   - gte: { hits.hits.2.inner_hits.sub_hits.hits.hits.1._seq_no: 0 }
   - gte: { hits.hits.2.inner_hits.sub_hits.hits.hits.1._primary_term: 1 }
+---
+"Test collapse with inner_hits and missing name fails":
+  - skip:
+      version: " - 8.12.99"
+      reason: fixed in 8.13
+  - do:
+      catch: bad_request
+      search:
+        index: test
+        body:
+          collapse: { field: numeric_group, inner_hits: { size: 1 } }
+          sort: [{ sort: desc }]

server/src/main/java/org/elasticsearch/search/collapse/CollapseBuilder.java

@@ -102,11 +102,21 @@ private CollapseBuilder setField(String field) {
     }
 
     public CollapseBuilder setInnerHits(InnerHitBuilder innerHit) {
+        if (innerHit.getName() == null) {
+            throw new IllegalArgumentException("inner_hits must have a [name]; set the [name] field in the inner_hits definition");
+        }
         this.innerHits = Collections.singletonList(innerHit);
         return this;
     }
 
     public CollapseBuilder setInnerHits(List<InnerHitBuilder> innerHits) {
+        if (innerHits != null) {
+            for (InnerHitBuilder innerHit : innerHits) {
+                if (innerHit.getName() == null) {
+                    throw new IllegalArgumentException("inner_hits must have a [name]; set the [name] field in the inner_hits definition");
+                }
+            }
+        }
         this.innerHits = innerHits;
         return this;
     }

server/src/test/java/org/elasticsearch/search/collapse/CollapseBuilderTests.java

@@ -174,7 +174,7 @@ public void testBuild() throws IOException {
                 null
             );
             when(searchExecutionContext.getFieldType("field")).thenReturn(numberFieldType);
-            builder.setInnerHits(new InnerHitBuilder());
+            builder.setInnerHits(new InnerHitBuilder().setName("field"));
             exc = expectThrows(IllegalArgumentException.class, () -> builder.build(searchExecutionContext));
             assertEquals(
                 exc.getMessage(),
@@ -194,7 +194,7 @@ public void testBuild() throws IOException {
 
             keywordFieldType = new KeywordFieldMapper.KeywordFieldType("field", false, true, Collections.emptyMap());
             when(searchExecutionContext.getFieldType("field")).thenReturn(keywordFieldType);
-            kbuilder.setInnerHits(new InnerHitBuilder());
+            kbuilder.setInnerHits(new InnerHitBuilder().setName("field"));
             exc = expectThrows(IllegalArgumentException.class, () -> builder.build(searchExecutionContext));
             assertEquals(
                 exc.getMessage(),