text-splitters: Add keep_separator arg to HTMLSemanticPreservingSplitter (#31588)

### Description
Add keep_separator arg to HTMLSemanticPreservingSplitter and pass value
to instance of RecursiveCharacterTextSplitter used under the hood.
### Issue
Documents returned by `HTMLSemanticPreservingSplitter.split_text(text)`
are defaulted to use separators at beginning of page_content. [See third
and fourth document in example output from how-to
guide](https://python.langchain.com/docs/how_to/split_html/#using-htmlsemanticpreservingsplitter):
```
[Document(metadata={'Header 1': 'Main Title'}, page_content='This is an introductory paragraph with some basic content.'),
 Document(metadata={'Header 2': 'Section 1: Introduction'}, page_content='This section introduces the topic'),
 Document(metadata={'Header 2': 'Section 1: Introduction'}, page_content='. Below is a list: First item Second item Third item with bold text and a link Subsection 1.1: Details This subsection provides additional details'),
 Document(metadata={'Header 2': 'Section 1: Introduction'}, page_content=". Here's a table: Header 1 Header 2 Header 3 Row 1, Cell 1 Row 1, Cell 2 Row 1, Cell 3 Row 2, Cell 1 Row 2, Cell 2 Row 2, Cell 3"),
 Document(metadata={'Header 2': 'Section 2: Media Content'}, page_content='This section contains an image and a video: ![image:example_image_link.mp4](example_image_link.mp4) ![video:example_video_link.mp4](example_video_link.mp4)'),
 Document(metadata={'Header 2': 'Section 3: Code Example'}, page_content='This section contains a code block: <code:html> <div> <p>This is a paragraph inside a div.</p> </div> </code>'),
 Document(metadata={'Header 2': 'Conclusion'}, page_content='This is the conclusion of the document.')]
```
### Dependencies
None

@ttrumper3

libs/text-splitters/langchain_text_splitters/html.py

@@ -10,10 +10,12 @@ from typing import (
     Dict,
     Iterable,
     List,
+    Literal,
     Optional,
     Sequence,
     Tuple,
     TypedDict,
+    Union,
     cast,
 )
 
@@ -535,6 +537,8 @@ class HTMLSemanticPreservingSplitter(BaseDocumentTransformer):
         preserve_parent_metadata (bool): Whether to pass through parent document
             metadata to split documents when calling
             ``transform_documents/atransform_documents()``.
+        keep_separator (Union[bool, Literal["start", "end"]]): Whether separators
+            should be at the beginning of a chunk, at the end, or not at all.
 
     Example:
         .. code-block:: python
@@ -584,6 +588,7 @@ class HTMLSemanticPreservingSplitter(BaseDocumentTransformer):
         allowlist_tags: Optional[List[str]] = None,
         denylist_tags: Optional[List[str]] = None,
         preserve_parent_metadata: bool = False,
+        keep_separator: Union[bool, Literal["start", "end"]] = True,
     ):
         """Initialize splitter."""
         try:
@@ -611,6 +616,7 @@ class HTMLSemanticPreservingSplitter(BaseDocumentTransformer):
         self._external_metadata = external_metadata or {}
         self._allowlist_tags = allowlist_tags
         self._preserve_parent_metadata = preserve_parent_metadata
+        self._keep_separator = keep_separator
         if allowlist_tags:
             self._allowlist_tags = list(
                 set(allowlist_tags + [header[0] for header in headers_to_split_on])
@@ -625,12 +631,15 @@ class HTMLSemanticPreservingSplitter(BaseDocumentTransformer):
         if separators:
             self._recursive_splitter = RecursiveCharacterTextSplitter(
                 separators=separators,
+                keep_separator=keep_separator,
                 chunk_size=max_chunk_size,
                 chunk_overlap=chunk_overlap,
             )
         else:
             self._recursive_splitter = RecursiveCharacterTextSplitter(
-                chunk_size=max_chunk_size, chunk_overlap=chunk_overlap
+                keep_separator=keep_separator,
+                chunk_size=max_chunk_size,
+                chunk_overlap=chunk_overlap,
             )
 
         if self._stopword_removal: