Add source line ranges to MarkdownHeaderTextSplitter chunks

### Submission checklist - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### 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 ### Feature Description I would like MarkdownHeaderTextSplitter to include source line range metadata for each emitted chunk, e.g. start_line and end_line, so downstream systems can trace chunks back to the original Markdown document. ### Use Case When building RAG or citation workflows over Markdown files, chunks need to be linked back to exact source line ranges for debugging, highlighting, review, and provenance. Currently users need to re-parse or maintain a separate mapping outside the splitter. ### Proposed Solution MarkdownHeaderTextSplitter could optionally add line range metadata to each Document returned by split_text, such as metadata["start_line"] and metadata["end_line"]. This would preserve backward compatibility while enabling source-aware retrieval and citation workflows. ### Alternatives Considered _No response_ ### Additional Context Originally filed as #38108, but it was closed because it was not submitted through the issue template.