RecursiveJsonSplitter - add optional `length_function` parameter to override chunk size measurement fn

### Submission checklist - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### 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 ### Feature Description An optional `length_function` init parameter would be useful in `RecursiveJsonSplitter`, overriding the built-in `_json_size` (`len(json.dumps(data))`) when provided. ### Use Case The `TextSplitter` abstract base class includes a `length_function` init parameter for optionally overriding the default of `len`; allowing any `TextSplitter` class to have its chunk measurement function overridden, with for example a tokenizer counting function. The same would be useful for `RecursiveJsonSplitter` for the same reasoning as `TextSplitter`. ### Proposed Solution In `TextSplitter` the `length_function` parameter is defined as follows: ```python length_function: Callable[[str], int] = len ``` I imagine a similar signature for `RecursiveJsonSplitter` could be: ```python length_function: Callable[[dict[str, Any]], int] | None = None ``` Where it defaults to `self._json_size` when `None`: ```python class RecursiveJsonSplitter: def __init__( self, *, length_funct…