Semantic text with legacy format cannot store term vectors (#119690)

This change fixes the store option of the underlying sparse vector field for semantic text.
For legacy indices, the store option should remain false since changing this option on an existing index is not allowed and that term vectors are already stored in _source.

Author: jimczi

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/mapper/SemanticTextFieldMapper.java

@@ -858,7 +858,7 @@ private static NestedObjectMapper.Builder createChunksField(
         );
         chunksField.dynamic(ObjectMapper.Dynamic.FALSE);
         if (modelSettings != null) {
-            chunksField.add(createEmbeddingsField(indexSettings.getIndexVersionCreated(), modelSettings));
+            chunksField.add(createEmbeddingsField(indexSettings.getIndexVersionCreated(), modelSettings, useLegacyFormat));
         }
         if (useLegacyFormat) {
             var chunkTextField = new KeywordFieldMapper.Builder(TEXT_FIELD, indexVersionCreated).indexed(false).docValues(false);
@@ -869,9 +869,13 @@ private static NestedObjectMapper.Builder createChunksField(
         return chunksField;
     }
 
-    private static Mapper.Builder createEmbeddingsField(IndexVersion indexVersionCreated, SemanticTextField.ModelSettings modelSettings) {
+    private static Mapper.Builder createEmbeddingsField(
+        IndexVersion indexVersionCreated,
+        SemanticTextField.ModelSettings modelSettings,
+        boolean useLegacyFormat
+    ) {
         return switch (modelSettings.taskType()) {
-            case SPARSE_EMBEDDING -> new SparseVectorFieldMapper.Builder(CHUNKED_EMBEDDINGS_FIELD).setStored(true);
+            case SPARSE_EMBEDDING -> new SparseVectorFieldMapper.Builder(CHUNKED_EMBEDDINGS_FIELD).setStored(useLegacyFormat == false);
             case TEXT_EMBEDDING -> {
                 DenseVectorFieldMapper.Builder denseVectorMapperBuilder = new DenseVectorFieldMapper.Builder(
                     CHUNKED_EMBEDDINGS_FIELD,

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/mapper/SemanticTextFieldMapperTests.java

@@ -536,7 +536,11 @@ private static void assertSemanticTextField(MapperService mapperService, String
             assertTrue(embeddingsFieldMapper.fieldType() == mapperService.mappingLookup().getFieldType(getEmbeddingsFieldName(fieldName)));
             assertThat(embeddingsMapper.fullPath(), equalTo(getEmbeddingsFieldName(fieldName)));
             switch (semanticFieldMapper.fieldType().getModelSettings().taskType()) {
-                case SPARSE_EMBEDDING -> assertThat(embeddingsMapper, instanceOf(SparseVectorFieldMapper.class));
+                case SPARSE_EMBEDDING -> {
+                    assertThat(embeddingsMapper, instanceOf(SparseVectorFieldMapper.class));
+                    SparseVectorFieldMapper sparseMapper = (SparseVectorFieldMapper) embeddingsMapper;
+                    assertEquals(sparseMapper.fieldType().isStored(), semanticTextFieldType.useLegacyFormat() == false);
+                }
                 case TEXT_EMBEDDING -> assertThat(embeddingsMapper, instanceOf(DenseVectorFieldMapper.class));
                 default -> throw new AssertionError("Invalid task type");
             }