bug(text-splitters): ExperimentalMarkdownSyntaxTextSplitter has O(n²) performance due to list.pop(0)

### Checked other resources - [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) - [x] langchain-text-splitters ### Related Issues / PRs None found. ### Reproduction Steps / Example Code (Python) ```python import time from langchain_text_splitters import ExperimentalMarkdownSyntaxTextSplitter splitter = ExperimentalMarkdownSyntaxTextSplitter() # Generate a large markdown document lines = [f"Line {i}: some content here\n" for i in range(50000)] large_md = "# Header\n" + "".join(lines) start = time.perf_counter() splitter.split_text(large_md) elapsed = time.perf_counter() - start print(f"50k lines: {elapsed:.2f}s") # On a typical machine this takes several seconds due to O(n²) behavior, # whereas O(n) should complete in milliseconds. ``` ### Error Message and Stack Trace (if applicable) No error — the issue is quadratic time complexity, not a crash. ### Description `ExperimentalMarkdownSyntaxTextSplitter.split_text()` uses `list.pop(0)` in a `while` loop to consume lines from the input (lines 395 and 445 in `markdown.py`). Since `list.pop(0)` is O(n) — it shifts all remaining elements left — doing this for every line makes the overall algorithm O(n²). For a 50,000-line markdown file, this means ~1.25 billion element shifts instead of 50,000 iteration steps. The fix is to use `collections.deque` with `popleft()`, which is O(1) per operation. The `raw_lines` variable is only used with: - `while raw_lines:` (truthiness check — works with deque) - `.pop(0)` …