Fix model settings propagation when merging semantic text fields (#129438)

Ensure that model settings are correctly set during mapping merges.
While this is not an issue currently, since the underlying embedding field is not customizable, this fix is required for correct behavior in #119967.

Author: jimczi

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

@@ -212,7 +212,6 @@ public static class Builder extends FieldMapper.Builder {
 
         private final Parameter<Map<String, String>> meta = Parameter.metaParam();
 
-        private MinimalServiceSettings resolvedModelSettings;
         private Function<MapperBuilderContext, ObjectMapper> inferenceFieldBuilder;
 
         public static Builder from(SemanticTextFieldMapper mapper) {
@@ -235,14 +234,18 @@ public Builder(
             super(name);
             this.modelRegistry = modelRegistry;
             this.useLegacyFormat = InferenceMetadataFieldsMapper.isEnabled(indexSettings.getSettings()) == false;
-            this.inferenceFieldBuilder = c -> createInferenceField(
-                c,
-                indexSettings.getIndexVersionCreated(),
-                useLegacyFormat,
-                resolvedModelSettings,
-                bitSetProducer,
-                indexSettings
-            );
+            this.inferenceFieldBuilder = c -> {
+                // Resolve the model setting from the registry if it has not been set yet.
+                var resolvedModelSettings = modelSettings.get() != null ? modelSettings.get() : getResolvedModelSettings(c, false);
+                return createInferenceField(
+                    c,
+                    indexSettings.getIndexVersionCreated(),
+                    useLegacyFormat,
+                    resolvedModelSettings,
+                    bitSetProducer,
+                    indexSettings
+                );
+            };
         }
 
         public Builder setInferenceId(String id) {
@@ -283,26 +286,26 @@ protected void merge(FieldMapper mergeWith, Conflicts conflicts, MapperMergeCont
             inferenceFieldBuilder = c -> mergedInferenceField;
         }
 
-        @Override
-        public SemanticTextFieldMapper build(MapperBuilderContext context) {
-            if (useLegacyFormat && copyTo.copyToFields().isEmpty() == false) {
-                throw new IllegalArgumentException(CONTENT_TYPE + " field [" + leafName() + "] does not support [copy_to]");
-            }
-            if (useLegacyFormat && multiFieldsBuilder.hasMultiFields()) {
-                throw new IllegalArgumentException(CONTENT_TYPE + " field [" + leafName() + "] does not support multi-fields");
+        /**
+         * Returns the {@link MinimalServiceSettings} defined in this builder if set;
+         * otherwise, resolves and returns the settings from the registry.
+         */
+        private MinimalServiceSettings getResolvedModelSettings(MapperBuilderContext context, boolean logWarning) {
+            if (context.getMergeReason() == MapperService.MergeReason.MAPPING_RECOVERY) {
+                // the model registry is not available yet
+                return null;
             }
-
-            if (context.getMergeReason() != MapperService.MergeReason.MAPPING_RECOVERY && modelSettings.get() == null) {
-                try {
-                    /*
-                     * If the model is not already set and we are not in a recovery scenario, resolve it using the registry.
-                     * Note: We do not set the model in the mapping at this stage. Instead, the model will be added through
-                     * a mapping update during the first ingestion.
-                     * This approach allows mappings to reference inference endpoints that may not yet exist.
-                     * The only requirement is that the referenced inference endpoint must be available at the time of ingestion.
-                     */
-                    resolvedModelSettings = modelRegistry.getMinimalServiceSettings(inferenceId.get());
-                } catch (ResourceNotFoundException exc) {
+            try {
+                /*
+                 * If the model is not already set and we are not in a recovery scenario, resolve it using the registry.
+                 * Note: We do not set the model in the mapping at this stage. Instead, the model will be added through
+                 * a mapping update during the first ingestion.
+                 * This approach allows mappings to reference inference endpoints that may not yet exist.
+                 * The only requirement is that the referenced inference endpoint must be available at the time of ingestion.
+                 */
+                return modelRegistry.getMinimalServiceSettings(inferenceId.get());
+            } catch (ResourceNotFoundException exc) {
+                if (logWarning) {
                     /* We allow the inference ID to be unregistered at this point.
                      * This will delay the creation of sub-fields, so indexing and querying for this field won't work
                      * until the corresponding inference endpoint is created.
@@ -315,8 +318,22 @@ public SemanticTextFieldMapper build(MapperBuilderContext context) {
                         inferenceId.get()
                     );
                 }
-            } else {
-                resolvedModelSettings = modelSettings.get();
+                return null;
+            }
+        }
+
+        @Override
+        public SemanticTextFieldMapper build(MapperBuilderContext context) {
+            if (useLegacyFormat && copyTo.copyToFields().isEmpty() == false) {
+                throw new IllegalArgumentException(CONTENT_TYPE + " field [" + leafName() + "] does not support [copy_to]");
+            }
+            if (useLegacyFormat && multiFieldsBuilder.hasMultiFields()) {
+                throw new IllegalArgumentException(CONTENT_TYPE + " field [" + leafName() + "] does not support multi-fields");
+            }
+
+            var resolvedModelSettings = modelSettings.get();
+            if (modelSettings.get() == null) {
+                resolvedModelSettings = getResolvedModelSettings(context, true);
             }
 
             if (modelSettings.get() != null) {