Fix some tests

Author: kderusso

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

@@ -1187,19 +1187,25 @@ static DenseVectorFieldMapper.DenseVectorIndexOptions defaultDenseVectorIndexOpt
         IndexVersion indexVersionCreated,
         MinimalServiceSettings modelSettings
     ) {
+
+        if (modelSettings.dimensions() == null) {
+            return null; // Cannot determine default index options without dimensions
+        }
+
         DenseVectorFieldMapper.DenseVectorIndexOptions defaultIndexOptions = null;
 
         // As embedding models for text perform better with BBQ, we aggressively default semantic_text fields to use optimized index
         // options
         if (indexVersionCreated.onOrAfter(SEMANTIC_TEXT_DEFAULTS_TO_BBQ)
             || indexVersionCreated.between(SEMANTIC_TEXT_DEFAULTS_TO_BBQ_BACKPORT_8_X, IndexVersions.UPGRADE_TO_LUCENE_10_0_0)) {
+
             DenseVectorFieldMapper.DenseVectorIndexOptions defaultBbqHnswIndexOptions = defaultBbqHnswDenseVectorIndexOptions();
             defaultIndexOptions = defaultBbqHnswIndexOptions.validate(modelSettings.elementType(), modelSettings.dimensions(), false)
                 ? defaultBbqHnswIndexOptions
                 : null;
         }
 
-        // Older indices or those incompatible with BBQ will continue to use legacy defaults
+        // Older indices or those incompatible with BBQ will continue to use legacy defaults, we specify them to ensure they are serialized
         if (defaultIndexOptions == null) {
             defaultIndexOptions = legacyDefaultDenseVectorIndexOptions();
         }

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

@@ -399,7 +399,7 @@ public void testInvalidTaskTypes() {
                     useLegacyFormat
                 )
             );
-            assertThat(e.getMessage(), containsString("Failed to parse mapping: Wrong [task_type]"));
+            assertThat(e.getMessage(), containsString("Invalid task_type in model_settings"));
         }
     }
 
@@ -1133,7 +1133,7 @@ private MapperService mapperServiceForFieldWithModelSettings(
                 inferenceId,
                 modelSettings,
                 generateRandomChunkingSettings(),
-                randomSemanticTextIndexOptions(TaskType.TEXT_EMBEDDING),
+                modelSettings.taskType() == TaskType.TEXT_EMBEDDING ? randomSemanticTextIndexOptions(TaskType.TEXT_EMBEDDING) : null,
                 Map.of()
             ),
             XContentType.JSON