text-splitters: fix state persistence issue in ExperimentalMarkdownSyntaxTextSplitter (#28373)

- **Description:** 
This PR resolves an issue with the
`ExperimentalMarkdownSyntaxTextSplitter` class, which retains the
internal state across multiple calls to the `split_text` method. This
behaviour caused an unintended accumulation of chunks in `self`
variables, leading to incorrect outputs when processing multiple
Markdown files sequentially.

- Modified `libs\text-splitters\langchain_text_splitters\markdown.py` to
reset the relevant internal attributes at the start of each `split_text`
invocation. This ensures each call processes the input independently.
- Added unit tests in
`libs\text-splitters\tests\unit_tests\test_text_splitters.py` to verify
the fix and ensure the state does not persist across calls.

- **Issue:**  
Fixes [#26440](https://github.com/langchain-ai/langchain/issues/26440).

- **Dependencies:**
No additional dependencies are introduced with this change.


- [x] Unit tests were added to verify the changes.
- [x] Updated documentation where necessary.  
- [x] Ran `make format`, `make lint`, and `make test` to ensure
compliance with project standards.

---------

Co-authored-by: Angel Chen <angelchen396@gmail.com>
Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/text-splitters/langchain_text_splitters/markdown.py

@@ -324,6 +324,11 @@ class ExperimentalMarkdownSyntaxTextSplitter:
             chunks of the input text. If `return_each_line` is enabled, each line
             is returned as a separate `Document`.
         """
+        # Reset the state for each new file processed
+        self.chunks.clear()
+        self.current_chunk = Document(page_content="")
+        self.current_header_stack.clear()
+
         raw_lines = text.splitlines(keepends=True)
 
         while raw_lines: