`HTMLHeaderTextSplitter` raises `ValueError` on init for the non-heading tags its splitter already supports

### 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 #40279 and #40298 are recent `text-splitters` reports, but neither touches header-tag handling. Nothing existing covers `headers_to_split_on` rejecting a tag outside `h1` to `h6`. ### Reproduction Steps / Example Code (Python) ```python from langchain_text_splitters import HTMLHeaderTextSplitter # `div` is one of the tags the splitting engine already assigns a fallback level to. HTMLHeaderTextSplitter(headers_to_split_on=[("h1", "Header 1"), ("div", "Div")]) ``` ### Error Message and Stack Trace (if applicable) ```shell Traceback (most recent call last): File "repro.py", line 4, in <module> HTMLHeaderTextSplitter(headers_to_split_on=[("h1", "Header 1"), ("div", "Div")]) File ".../langchain_text_splitters/html.py", line 175, in __init__ self.headers_to_split_on = sorted( ^^^^^^^ F…