Add assertion that all field mapper names are interned (#87238)

Follow up to #86301. Assert that all field names are interned so we don't
lose this optimization and fix `FieldAliasMapper.name` for it not to trip.

server/src/main/java/org/elasticsearch/index/mapper/FieldAliasMapper.java

@@ -36,7 +36,7 @@ public static class Names {
 
     public FieldAliasMapper(String simpleName, String name, String path) {
         super(simpleName);
-        this.name = name;
+        this.name = Mapper.internFieldName(name);
         this.path = path;
     }
 

server/src/main/java/org/elasticsearch/index/mapper/MappingLookup.java

@@ -186,6 +186,22 @@ private MappingLookup(
         this.indexTimeScriptMappers = List.copyOf(indexTimeScriptMappers);
 
         runtimeFields.stream().flatMap(RuntimeField::asMappedFieldTypes).map(MappedFieldType::name).forEach(this::validateDoesNotShadow);
+        assert assertMapperNamesInterned(this.fieldMappers, this.objectMappers);
+    }
+
+    private static boolean assertMapperNamesInterned(Map<String, Mapper> mappers, Map<String, ObjectMapper> objectMappers) {
+        mappers.forEach(MappingLookup::assertNamesInterned);
+        objectMappers.forEach(MappingLookup::assertNamesInterned);
+        return true;
+    }
+
+    private static void assertNamesInterned(String name, Mapper mapper) {
+        assert name == name.intern();
+        assert mapper.name() == mapper.name().intern();
+        assert mapper.simpleName() == mapper.simpleName().intern();
+        if (mapper instanceof ObjectMapper) {
+            ((ObjectMapper) mapper).mappers.forEach(MappingLookup::assertNamesInterned);
+        }
     }
 
     /**

server/src/test/java/org/elasticsearch/search/suggest/AbstractSuggestionBuilderTestCase.java

@@ -266,7 +266,8 @@ public void testBuildWithUnmappedField() {
 
     protected MappedFieldType mockFieldType(String fieldName) {
         MappedFieldType fieldType = mock(MappedFieldType.class);
-        when(fieldType.name()).thenReturn(fieldName);
+        when(fieldType.name()).thenReturn(fieldName.intern()); // intern field name to not trip assertions that ensure all field names are
+                                                               // interned
         NamedAnalyzer searchAnalyzer = new NamedAnalyzer("fieldSearchAnalyzer", AnalyzerScope.INDEX, new SimpleAnalyzer());
         TextSearchInfo tsi = new TextSearchInfo(TextFieldMapper.Defaults.FIELD_TYPE, null, searchAnalyzer, searchAnalyzer);
         when(fieldType.getTextSearchInfo()).thenReturn(tsi);