Add Semantic Text Default BBQ Tests For 8.x Index Versions (elastic#127862)

Author: Mikep86

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

@@ -1185,6 +1185,35 @@ public void testDefaultIndexOptions() throws IOException {
         );
         assertSemanticTextField(mapperService, "field", true, null, defaultDenseVectorIndexOptions());
 
+        // 8.x index versions that use backported default BBQ set default BBQ index options as expected
+        mapperService = createMapperService(fieldMapping(b -> {
+            b.field("type", "semantic_text");
+            b.field("inference_id", "another_inference_id");
+            b.startObject("model_settings");
+            b.field("task_type", "text_embedding");
+            b.field("dimensions", 100);
+            b.field("similarity", "cosine");
+            b.field("element_type", "float");
+            b.endObject();
+        }), useLegacyFormat, IndexVersions.SEMANTIC_TEXT_DEFAULTS_TO_BBQ_BACKPORT_8_X, IndexVersions.UPGRADE_TO_LUCENE_10_0_0);
+        assertSemanticTextField(mapperService, "field", true, null, SemanticTextFieldMapper.defaultSemanticDenseIndexOptions());
+
+        // Previous 8.x index versions do not set BBQ index options
+        mapperService = createMapperService(fieldMapping(b -> {
+            b.field("type", "semantic_text");
+            b.field("inference_id", "another_inference_id");
+            b.startObject("model_settings");
+            b.field("task_type", "text_embedding");
+            b.field("dimensions", 100);
+            b.field("similarity", "cosine");
+            b.field("element_type", "float");
+            b.endObject();
+        }),
+            useLegacyFormat,
+            IndexVersions.INFERENCE_METADATA_FIELDS_BACKPORT,
+            IndexVersionUtils.getPreviousVersion(IndexVersions.SEMANTIC_TEXT_DEFAULTS_TO_BBQ_BACKPORT_8_X)
+        );
+        assertSemanticTextField(mapperService, "field", true, null, defaultDenseVectorIndexOptions());
     }
 
     @Override