Related to #31887 (custom header pattern support)

### 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 Related to #31887 (custom header pattern support) ### Reproduction Steps / Example Code (Python) ```python from langchain_text_splitters import MarkdownHeaderTextSplitter splitter = MarkdownHeaderTextSplitter( headers_to_split_on=[("**", "Header 1"), ("***", "Header 2")], custom_header_patterns={"**": 1, "***": 2}, strip_headers=False, ) for doc in splitter.split_text("**H1**\n***H2***\nbody\n"): print(repr(doc.page_content), doc.metadata) # Actual output: # '**H1**' {'Header 1': 'H1'} # '***H2***\nbody' {'Header 1': 'H1', 'Header 2': 'H2'} splitter = MarkdownHeaderTextSplitter( headers_to_split_on=[("#", "Header 1"), ("##", "Header 2")], strip_headers=False, ) for doc in splitter.split_text("# H1\n## H2\nbody\n"): print(repr(doc.page_content), doc.metadata) # Comparable standard-header output: #…