Prevent field_caps from failing when can-match fails

`FieldCapabilitiesFetcher` performs a can-match in order to quickly
return an empty response if no shard can match. However, if can-match
fails for some reason, it can cause the field capabilities request to fail.

An example of that is when a semantic query is used as filter. can-match
will fail as it won't be able to expand the inference results of the query.

In cases like that, it makes no sense to fail the field capabilities request.
Instead, we should treat can-match as returning `true` to proceed.

This change does that by following suit with other callers of can-match.

Fixes #116106

Author: dimitris-athanasiou

server/src/main/java/org/elasticsearch/action/fieldcaps/FieldCapabilitiesFetcher.java

@@ -249,12 +249,17 @@ private static boolean canMatchShard(
         QueryBuilder indexFilter,
         long nowInMillis,
         SearchExecutionContext searchExecutionContext
-    ) throws IOException {
+    ) {
         assert alwaysMatches(indexFilter) == false : "should not be called for always matching [" + indexFilter + "]";
         assert nowInMillis != 0L;
         ShardSearchRequest searchRequest = new ShardSearchRequest(shardId, nowInMillis, AliasFilter.EMPTY);
         searchRequest.source(new SearchSourceBuilder().query(indexFilter));
-        return SearchService.queryStillMatchesAfterRewrite(searchRequest, searchExecutionContext);
+        try {
+            return SearchService.queryStillMatchesAfterRewrite(searchRequest, searchExecutionContext);
+        } catch (Exception e) {
+            // treat as if shard is still a potential match
+            return true;
+        }
     }
 
     private static boolean alwaysMatches(QueryBuilder indexFilter) {

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

@@ -59,6 +59,7 @@ public Set<NodeFeature> getTestFeatures() {
                 SemanticTextFieldMapper.SEMANTIC_TEXT_DELETE_FIX,
                 SemanticTextFieldMapper.SEMANTIC_TEXT_ZERO_SIZE_FIX,
                 SemanticTextFieldMapper.SEMANTIC_TEXT_ALWAYS_EMIT_INFERENCE_ID_FIX,
+                SemanticTextFieldMapper.SEMANTIC_TEXT_FILTER_FIELD_CAPS_FIX,
                 SemanticTextFieldMapper.SEMANTIC_TEXT_SKIP_INFERENCE_FIELDS,
                 SEMANTIC_TEXT_HIGHLIGHTER,
                 SEMANTIC_MATCH_QUERY_REWRITE_INTERCEPTION_SUPPORTED,

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

@@ -136,6 +136,7 @@ public class SemanticTextFieldMapper extends FieldMapper implements InferenceFie
     public static final NodeFeature SEMANTIC_TEXT_ALWAYS_EMIT_INFERENCE_ID_FIX = new NodeFeature(
         "semantic_text.always_emit_inference_id_fix"
     );
+    public static final NodeFeature SEMANTIC_TEXT_FILTER_FIELD_CAPS_FIX = new NodeFeature("semantic_text.filter_field_caps_fix");
     public static final NodeFeature SEMANTIC_TEXT_HANDLE_EMPTY_INPUT = new NodeFeature("semantic_text.handle_empty_input");
     public static final NodeFeature SEMANTIC_TEXT_SKIP_INFERENCE_FIELDS = new NodeFeature("semantic_text.skip_inference_fields");
     public static final NodeFeature SEMANTIC_TEXT_BIT_VECTOR_SUPPORT = new NodeFeature("semantic_text.bit_vector_support");

x-pack/plugin/inference/src/yamlRestTest/resources/rest-api-spec/test/inference/100_semantic_text_field_caps.yml

@@ -0,0 +1,56 @@
+setup:
+  - requires:
+      cluster_features: "semantic_text.filter_field_caps_fix"
+      reason: "fixed bug with semantic query filtering in field_caps (#116106)"
+
+  - 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:
+          mappings:
+            properties:
+              semantic_text_field:
+                type: semantic_text
+                inference_id: sparse-inference-id
+              std_text_field:
+                type: text
+
+  - do:
+      index:
+        index: test-index
+        id: doc_1
+        body:
+          semantic_text_field: "This is a story about a cat and a dog."
+          std_text_field: "some text"
+        refresh: true
+
+---
+"Field caps with semantic_text filter does not fail":
+  - do:
+      field_caps:
+        index: test-index
+        fields: "*"
+        body:
+          index_filter:
+            semantic:
+              field: "semantic_text_field"
+              query: "test"
+
+  - match: { indices: [ "test-index" ] }
+  - match: { fields.semantic_text_field.text.searchable: true }
+  - match: { fields.std_text_field.text.searchable: true }