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/ingestion/task/indexdoc/process.go

@@ -77,7 +77,7 @@ func ProcessChunksForPipeline(
 
 	for _, ck := range chunks {
 		ck["doc_id"] = docID
-		ck["kb_id"] = []string{kbID}
+		ck["kb_id"] = kbID
 		ck["docnm_kwd"] = docName
 		ck["create_time"] = timeStr
 		ck["create_timestamp_flt"] = timestamp