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/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