text-splitters: `RecursiveCharacterTextSplitter` skips `strip_whitespace` for pieces it cannot split further

### 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 Searched for "RecursiveCharacterTextSplitter strip_whitespace" in open and closed issues and PRs; no existing report. Supersedes #40291, which the triage bot closed for missing an issue type. The fix is ready in #40294. ### Reproduction Steps / Example Code (Python) ```python from langchain_text_splitters import RecursiveCharacterTextSplitter splitter = RecursiveCharacterTextSplitter(separators=["\n", " "], chunk_size=5, chunk_overlap=0) print(splitter.split_text("hi supercalifragilistic\nok")) # ['hi', ' supercalifragilistic', 'ok'] <- note the leading space splitter = RecursiveCharacterTextSplitter(separators=[" "], chunk_size=3, chunk_overlap=0) print(splitter.split_text("ab ab ab")) # ['ab', ' ab', ' ab'] ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description `strip_whitespace` defaults to …