refactor(ingestion/task): emit kb_id as a single string in ProcessChunksForPipeline (#17802)

## Summary
- `ProcessChunksForPipeline` now sets `kb_id` to a plain string instead
of `[]string{kbID}`, removing an index-physical array shape from the
ingestion domain.
- Stored documents are byte-identical: Elasticsearch overrides `kb_id`
with `datasetID` on write, and Infinity's `transformChunkFields` already
accepts a plain string.
- Infinity is intentionally left unchanged — `service/chunk` paths still
feed `kb_id` as `[]string`, and Infinity handles both forms. The
`dataset` artifact merge (`dataset_artifact_service.go`) is out of scope
for this step.
- Unit assertion updated to expect a string.

## Scope / non-goals
This is the smallest first step (T1) of the index-schema leak cleanup
tracked in #17371. It does **not** move the other leaks (`docnm_kwd`,
`create_timestamp_flt`, position ints) to the engine boundary — those
are later steps behind a read-back golden test.

## Test plan
- `go test ./internal/ingestion/task/indexdoc/...` passes.
- The two `task` "Real" integration tests fail identically on a clean
tree (environment lacks real embedding/parsing); they are pre-existing,
unrelated to this change.

🤖 Generated with [CodeBuddy Code](https://cnb.cool/codebuddy)

internal/engine/elasticsearch/chunk_readback_integration_test.go

@@ -0,0 +1,110 @@
+// 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.
+
+//go:build integration
+
+package elasticsearch
+
+import (
+	"context"
+	"reflect"
+	"testing"
+
+	"ragflow/internal/common"
+	"ragflow/internal/server/config"
+)
+
+// TestInsertChunks_ReadBackSuffixedFields is the T0 integration-tier read-back
+// baseline (issue #17371): it writes a chunk to a real Elasticsearch instance
+// and reads it back, asserting the stored document keeps the index-physical
+// field names the ingestion pipeline emits. This catches regressions that the
+// unit-tier test (which only inspects the request body) cannot — e.g. an ES
+// mapping coercion that changes the stored value.
+//
+// Requires a running Elasticsearch; set ES_TEST=1 to run.
+func TestInsertChunks_ReadBackSuffixedFields(t *testing.T) {
+	if common.GetEnv(common.EnvESTest) != "1" {
+		t.Skip("Skipping ES integration test; set ES_TEST=1 to run")
+	}
+
+	engine, err := NewEngine(getESTestConfig())
+	if err != nil {
+		t.Fatalf("NewEngine: %v", err)
+	}
+
+	ctx := context.Background()
+	baseName := "ragflow_chunk_readback_test"
+	datasetID := "kb-1"
+	chunkID := "readback-chunk-1"
+	chunk := map[string]interface{}{
+		"doc_id":               "doc-1",
+		"id":                   chunkID,
+		"kb_id":                "producer-kb-1",
+		"docnm_kwd":            "sample.md",
+		"content_with_weight":  "hello world",
+		"create_timestamp_flt": float64(123.0),
+		"question_kwd":         []string{"q1", "q2"},
+		"important_kwd":        []string{"k1"},
+		"page_num_int":         int(1),
+		"position_int":         int(2),
+	}
+
+	if _, err := engine.InsertChunks(ctx, []map[string]interface{}{chunk}, baseName, datasetID); err != nil {
+		t.Fatalf("InsertChunks: %v", err)
+	}
+
+	got, err := engine.GetChunk(ctx, baseName, chunkID, []string{datasetID})
+	if err != nil {
+		t.Fatalf("GetChunk: %v", err)
+	}
+	stored, ok := got.(map[string]interface{})
+	if !ok {
+		t.Fatalf("GetChunk returned %T, want map[string]interface{}", got)
+	}
+
+	assertStoredField(t, stored, "docnm_kwd", "sample.md")
+	assertStoredField(t, stored, "content_with_weight", "hello world")
+	assertStoredField(t, stored, "create_timestamp_flt", float64(123.0))
+	assertStoredField(t, stored, "page_num_int", float64(1))
+	// The input kb_id ("producer-kb-1") differs from datasetID ("kb-1"); the
+	// stored value must equal datasetID, proving InsertChunks overrides it.
+	assertStoredField(t, stored, "kb_id", datasetID)
+	if v, ok := stored["question_kwd"]; !ok || !reflect.DeepEqual(v, []interface{}{"q1", "q2"}) {
+		t.Errorf("stored question_kwd = %#v, want [q1 q2]", v)
+	}
+}
+
+// getESTestConfig builds an Elasticsearch config for integration tests from
+// the environment, falling back to localhost defaults. It is kept local to this
+// file so the read-back test does not depend on kg_test.go (whose getTestConfig
+// is an unrelated main-branch compile fix tracked separately).
+func getESTestConfig() config.ElasticsearchConfig {
+	hosts := common.GetEnv(common.EnvESHost)
+	if hosts == "" {
+		hosts = "http://localhost:1200"
+	}
+	username := common.GetEnv(common.EnvESUsername)
+	if username == "" {
+		username = "elastic"
+	}
+	password := common.GetEnv(common.EnvESPassword)
+	if password == "" {
+		password = "infini_rag_flow"
+	}
+	return config.ElasticsearchConfig{
+		Hosts:    hosts,
+		Username: username,
+		Password: password,
+	}
+}