Refactor: refactor dataflow_service.go and guard nats message re-delivery (#16826)

### Summary

1. refactor dataflow_service.go 
2. guard nats message re-delivery
3. support document parse cancelling & re-run

internal/engine/engine.go

@@ -76,6 +76,9 @@ type DocEngine interface {
 	// GetType returns the engine type
 	GetType() string
 
+	// SupportsPageRank reports whether the engine supports dataset-level pagerank.
+	SupportsPageRank() bool
+
 	// FilterDocIdsByMetaPushdown runs a metadata filter directly against
 	// the doc metadata index, returning matching doc IDs or nil if push-down
 	// is not supported (caller should fall back to in-memory filtering).