CharacterTextSplitter/RecursiveCharacterTextSplitter corrupt chunk content when merging with a non-lookaround regex separator and keep_separator=False

### 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 This is a long-standing, recurring problem with a partial fix already merged, and I want to be explicit about that history: - #31137 (merged) added detection for zero-width "lookaround" regex separators ((?=, (?<=, (?!, (?<!) in CharacterTextSplitter.split_text, forcing merge_sep="" for those specifically -- but its own comment ("Detect zero-width lookaround so we never re-insert it") scopes the fix explicitly to lookaround patterns. Any ordinary *consuming* regex separator (e.g. \s+, [,;]+, \n{2,}) still falls through to using the raw pattern string as the literal merge separator -- the exact bug #31137 fixed, just for a narrower trigger. - RecursiveCharacterTextSplitter never received even that narrow fix -- its merge path (_split_text in character.py) has no lookaround detection at all. Prior reports of this …