[Bug]: HierarchicalNodeParser async methods return the documents unsplit

Finding: In an external report against llama-index-core 0.14.25 (Python 3.12.3, Linux), HierarchicalNodeParser reportedly returns the input Document unsplit through its async entry points (aget_nodes_from_documents, acall, IngestionPipeline.arun), while the sync paths build the hierarchy [https://github.com/run-llama/llama_index/issues/23444]. This is an external report only; KFA did not reproduce it. Reported symptom: using a synthetic document with chunk_sizes=[512, 128], the reporter says sync get_nodes_from_documents and IngestionPipeline.run produced 69 TextNodes, while the async methods produced 1 node of type Document, and the first node was the input document object itself. No exception is raised, so the failure is silent: an async ingestion would index whole documents with no parent/child relationships, leaving get_leaf_nodes and AutoMergingRetriever without a hierarchy to use, per the reporter [https://github.com/run-llama/llama_index/issues/23444]. Reporter's hypothesis on cause (not maintainer-confirmed): the parser overrides only the sync get_nodes_from_documents; its _parse_nodes is marked as an unused abstract method and returns the input nodes unchanged; the inherited async methods go through _aparse_nodes, which falls back to _parse_nodes, so the recursive chunking is never reached. The reporter also notes the test file has no async case [https://github.com/run-llama/llama_index/issues/23444]. Diagnostic value: if an async pipeline using HierarchicalNodeParser yields Document objects rather than TextNode chunks, or node counts differ from the sync run, this report matches that signature. Compare node counts and types between sync and async runs on the same input. Status: the issue was open with no comments or maintainer response in the supplied content. The reporter proposed a fix (override the async method to run the same recursion, or move the recursion into _parse_nodes) and offered a PR, but no PR, fix commit, or released fix was supplied. Do not assume any released version fixes this. Affected versions other than 0.14.25 are unknown. Limitations: single report, no independent reproduction, comments coverage may be incomplete, and the cause analysis refers to main at commit 6dd2f3cdd as read by the reporter.