Fix - Requesting _inference_fields when using legacy format causes shard failure

docs/changelog/121720.yaml

@@ -0,0 +1,6 @@
+pr: 121720
+summary: Fix - Requesting `_inference_fields` when using legacy format causes shard
+  failure
+area: Relevance
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/search/fetch/subphase/FetchFieldsPhase.java

@@ -71,6 +71,7 @@ public FetchSubPhaseProcessor getProcessor(FetchContext fetchContext) {
                         .stream()
                         .filter(
                             fieldAndFormat -> (searchExecutionContext.isMetadataField(fieldAndFormat.field) == false
+                                || searchExecutionContext.fieldExistsInIndex(fieldAndFormat.field) == false
                                 || searchExecutionContext.getFieldType(fieldAndFormat.field).isStored() == false
                                 || IdFieldMapper.NAME.equals(fieldAndFormat.field)
                                 || SourceFieldMapper.NAME.equals(fieldAndFormat.field))
@@ -87,6 +88,7 @@ public FetchSubPhaseProcessor getProcessor(FetchContext fetchContext) {
                     continue;
                 }
                 if (searchExecutionContext.isMetadataField(fieldAndFormat.field)
+                    && searchExecutionContext.fieldExistsInIndex(fieldAndFormat.field)
                     && searchExecutionContext.getFieldType(fieldAndFormat.field).isStored()) {
                     fetchContextMetadataFields.add(fieldAndFormat);
                 }

x-pack/plugin/inference/src/yamlRestTest/resources/rest-api-spec/test/inference/40_semantic_text_query.yml

@@ -1003,3 +1003,60 @@ setup:
   - length: { aggregations.agg_ids.buckets: 1 }
   - match: { aggregations.agg_ids.buckets.0.key: "doc_1" }
   - match: { aggregations.agg_ids.buckets.0.doc_count: 1 }
+
+---
+"Querying_inference_fields with semantic_text legacy format":
+  - do:
+      inference.put:
+        task_type: sparse_embedding
+        inference_id: sparse-inference-id
+        body: >
+          {
+            "service": "test_service",
+            "service_settings": {
+              "model": "my_model",
+              "api_key": "abc64"
+            },
+            "task_settings": {
+            }
+          }
+
+  - do:
+      indices.create:
+        index: test-index
+        body:
+          settings:
+            index:
+              mapping:
+                semantic_text:
+                  use_legacy_format: true
+          mappings:
+            properties:
+              inference_field:
+                type: semantic_text
+                inference_id: sparse-inference-id
+              source_field:
+                type: text
+                copy_to: inference_field
+
+  - do:
+      index:
+        index: test-index
+        id: doc_1
+        body:
+          inference_field: "test value"
+          source_field: "source value"
+        refresh: true
+
+  - do:
+      search:
+        index: test-index
+        body:
+          fields: [ _inference_fields ]
+          query:
+            match_all: { }
+
+  - match: { hits.total.value: 1 }
+  - match: { hits.total.relation: eq }
+  - match: { hits.hits.0._source.source_field: "source value" }
+  - match: { hits.hits.0._source.inference_field.text: "test value" }