text-splitters: `MarkdownHeaderTextSplitter` keeps the closing `##` sequence in header metadata

### Submission checklist - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [x] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs Searched for "MarkdownHeaderTextSplitter closing hashes" / "trailing #" in open and closed issues and PRs; #40071 (tabs and NBSP dropped) and its PRs touch the same splitter but a different problem. Supersedes #40290, which the triage bot closed for missing an issue type. The fix is ready in #40293. ### Reproduction Steps / Example Code (Python) ```python from langchain_text_splitters import MarkdownHeaderTextSplitter splitter = MarkdownHeaderTextSplitter([("#", "Header 1"), ("##", "Header 2")]) for doc in splitter.split_text("# Title ##\nIntro.\n## Section ###\nBody."): print(doc.metadata) # {'Header 1': 'Title ##'} # {'Header 1': 'Title ##', 'Header 2': 'Section ###'} ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description CommonMark ATX headings may carry an optional closing sequence of `#`…