feat: support SelectFields override in DocEngine for KG-specific queries (#15684)

## Summary

Both ES and Infinity engines now respect `SearchRequest.SelectFields`,
allowing callers to specify output columns for KG
entity/relation/community queries instead of the default chunk columns.

### Changes

- **`internal/engine/elasticsearch/chunk.go`**: Added `SelectFields`
override after default `outputColumns`
- **`internal/engine/infinity/chunk.go`**: Added `SelectFields` override
after default `outputColumns`
- **`internal/engine/elasticsearch/kg_test.go`** (new): Integration test
(skipped unless `ES_TEST=1`)

### Usage

```go
result, err := docEngine.Search(ctx, \&types.SearchRequest{
    KbIDs:        kbIDs,
    SelectFields: []string{entity_kwd, entity_type_kwd, rank_flt, n_hop_with_weight},
    Filter:       map[string]interface{}{knowledge_graph_kwd: entity},
})
```

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

internal/engine/elasticsearch/chunk.go

@@ -771,6 +771,11 @@ func (e *elasticsearchEngine) searchUnified(ctx context.Context, req *types.Sear
 		}
 	}
 
+	// Allow caller to override output columns (used by KG search, etc.)
+	if len(req.SelectFields) > 0 {
+		outputColumns = req.SelectFields
+	}
+
 	hasTextMatch := false
 	hasVectorMatch := false
 	var matchText *types.MatchTextExpr

internal/engine/elasticsearch/kg_test.go

@@ -0,0 +1,94 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package elasticsearch
+
+import (
+	"context"
+	"os"
+	"testing"
+
+	"ragflow/internal/engine/types"
+)
+
+// TestKGSearchSelectFields verifies that SelectFields overrides default output
+// columns when searching for knowledge graph entities.
+// Requires a running Elasticsearch instance and KG data indexed by Python task executor.
+// Set ES_TEST=1 to run.
+func TestKGSearchSelectFields(t *testing.T) {
+	if os.Getenv("ES_TEST") != "1" {
+		t.Skip("Skipping ES integration test; set ES_TEST=1 to run")
+	}
+
+	engine, err := NewEngine(getTestConfig())
+	if err != nil {
+		t.Fatalf("failed to create engine: %v", err)
+	}
+
+	// Search for KG entities using SelectFields
+	req := &types.SearchRequest{
+		IndexNames: []string{"ragflow_*"},
+		KbIDs:      []string{},
+		Filter: map[string]interface{}{
+			"knowledge_graph_kwd": "entity",
+		},
+		SelectFields: []string{"entity_kwd", "entity_type_kwd", "rank_flt"},
+		Limit:        10,
+	}
+
+	result, err := engine.Search(context.Background(), req)
+	if err != nil {
+		t.Fatalf("search failed: %v", err)
+	}
+
+	// Verify returned chunks contain only allowed fields
+	allowedFields := map[string]bool{
+		"entity_kwd":      true,
+		"entity_type_kwd": true,
+		"rank_flt":        true,
+		"_score":          true,
+	}
+
+	for i, chunk := range result.Chunks {
+		for key := range chunk {
+			if !allowedFields[key] {
+				t.Errorf("chunk[%d] contains unexpected field: %s (allowed: entity_kwd, entity_type_kwd, rank_flt)", i, key)
+			}
+		}
+	}
+}
+
+// getTestConfig returns a minimal ES config for testing.
+// Reads from environment or uses defaults pointing to localhost.
+func getTestConfig() map[string]interface{} {
+	hosts := os.Getenv("ES_HOSTS")
+	if hosts == "" {
+		hosts = "http://localhost:1200"
+	}
+	username := os.Getenv("ES_USERNAME")
+	if username == "" {
+		username = "elastic"
+	}
+	password := os.Getenv("ES_PASSWORD")
+	if password == "" {
+		password = "infini_rag_flow"
+	}
+	return map[string]interface{}{
+		"hosts":    []string{hosts},
+		"username": username,
+		"password": password,
+	}
+}

internal/engine/infinity/chunk.go

@@ -654,6 +654,11 @@ func (e *infinityEngine) Search(ctx context.Context, req *types.SearchRequest) (
 		outputColumns = convertSelectFields(outputColumns)
 	}
 
+	// Allow caller to override output columns (used by KG search, etc.)
+	if len(req.SelectFields) > 0 {
+		outputColumns = convertSelectFields(req.SelectFields)
+	}
+
 	hasTextMatch := false
 	hasVectorMatch := false
 	var matchText *types.MatchTextExpr