text-splitters: MarkdownHeaderTextSplitter silently drops tabs and NBSP (including inside fenced code)

### 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 * Prior CLI filing #40066 was auto-closed for missing issue type; re-filing via web template. * ExperimentalMarkdownSyntaxTextSplitter documents retaining exact whitespace as a re-implementation of this splitter. * Ready PR: https://github.com/langchain-ai/langchain/pull/40068 ### Reproduction Steps / Example Code (Python) ```python from langchain_text_splitters import MarkdownHeaderTextSplitter s = MarkdownHeaderTextSplitter(headers_to_split_on=[("#", "h1")]) docs = s.split_text("# Title\n\n\ndef foo():\n\treturn 1\n\n") print(repr(docs[0].page_content)) # actual: tab indent destroyed # expected: tab before return preserved docs2 = s.split_text("# Title\n\nhello\tworld") print(repr(docs2[0].page_content)) # actual: 'helloworld' # expected: 'hello\tworld' docs3 = s.split_text("# Title\n\nhello\u00a0world") print(repr(docs3[…