Support for Custom Metadata Hydrators in Text Splitters

### Checked other resources - [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. ### Feature Description ## 🚀 Feature Request: Support for Custom Metadata Hydrators in Text Splitters ### Summary Currently, LangChain’s text splitters (`CharacterTextSplitter`, `RecursiveCharacterTextSplitter`, etc.) allow splitting documents into chunks with basic metadata propagation (like `source`, `page_content`, etc.). However, there is no way to dynamically **inject or enrich metadata** during the splitting process — e.g. by adding custom fields such as `chunk_number`, `section_title`, or context-aware data derived from the splitting logic. I propose adding support for a **custom Metadata Hydrator** that can modify or extend document metadata during the split operation. --- ### Motivation When building Retrieval-Augmented Generation (RAG) pipelines or complex document indexing workflows, it’s often necessary to enrich metadata with contextual information about each chunk. Examples include: - Tracking `chunk_number` per document - Adding the name of the section or heading that the chunk originated from - Including dynamic tokens like the splitter’s position or semantic markers Right now, this requires post-processing the split results manually, which is inefficient and error-prone. --- ### Proposed Solution Introduce an optional `MetadataHydrator` interface and method like `.set_metadata_hydrator()` on all `TextSplitter` classes. #### Example API: ```python from langchain_text_splitters import CharacterTextSplitter, BaseMetadataHydrator class MetaDataCustomHydrator(BaseMetadataHydrator): def __init__(self, uploadedBy: str = 'us…