`TextSplitter` `chunk_overlap` is silently ignored unless `chunk_size` overflow occurs

### 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) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-cli - [ ] 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-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Reproduction Steps / Example Code (Python) ```python from langchain_text_splitters import RecursiveCharacterTextSplitter text = """ TechDocs Employee Handbook Chapter 1: Remote Work Policy Our company embraces flexible work arrangements to support work-life balance. Employees are permitted to work remotely up to 3 days per week, provided they maintain regular communication with their team and meet all performance expectations. Remote work requires manager approval and must not impact team collaboration or customer service quality. To work remotely, employees must have a suitable home office setup with reliable internet connection, appropriate workspace, and necessary equipment. The company provides a one-time stipend of $500 for home office setup. VPN access is mandatory for accessing company systems re…