RecursiveJsonSplitter produces chunks exceeding max_chunk_size for nested JSON

### 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 #37732,#37734 ### Reproduction Steps / Example Code (Python) ```python from langchain_text_splitters import RecursiveJsonSplitter import json splitter = RecursiveJsonSplitter(max_chunk_size=50) data = { "container": { "a": {"x": 1, "y": 2, "z": 3}, "b": "test", }, } chunks = splitter.split_json(data) for i, chunk in enumerate(chunks): size = len(json.dumps(chunk)) assert size <= 50, f"chunk {i} is {size}, exceeds max_chunk_size=50" ``` ### Error Message and Stack Trace (if applicable) ```shell AssertionError: chunk 0 is 59, exceeds max_chunk_size=50 ``` ### Description `split_json()` violates `max_chunk_size` (under tight constraints) when JSON has nested structure. Two bugs in [`_json_split()`](https://github.com/langchain-ai/langchain/blob/cdca311de2958e497cfd823a45ddb770d0ef94c0/libs/text-…