text-splitters[patch]: fix MarkdownHeaderTextSplitter fails to parse headers with non-printable characters (#20645)

Description: MarkdownHeaderTextSplitter Fails to Parse Headers with
non-printable characters. more #20643

The following is the official test case. Just replacing `# Foo\n\n` with
`\ufeff# Foo\n\n` will cause the test case to fail.

chunk metadata is empty

```python
def test_md_header_text_splitter_1() -> None:
    """Test markdown splitter by header: Case 1."""

    markdown_document = (
        "\ufeff# Foo\n\n"
        "    ## Bar\n\n"
        "Hi this is Jim\n\n"
        "Hi this is Joe\n\n"
        " ## Baz\n\n"
        " Hi this is Molly"
    )
    headers_to_split_on = [
        ("#", "Header 1"),
        ("##", "Header 2"),
    ]
    markdown_splitter = MarkdownHeaderTextSplitter(
        headers_to_split_on=headers_to_split_on,
    )
    output = markdown_splitter.split_text(markdown_document)
    expected_output = [
        Document(
            page_content="Hi this is Jim  \nHi this is Joe",
            metadata={"Header 1": "Foo", "Header 2": "Bar"},
        ),
        Document(
            page_content="Hi this is Molly",
            metadata={"Header 1": "Foo", "Header 2": "Baz"},
        ),
    ]
    assert output == expected_output
```

twitter: @coolbeevip

Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/text-splitters/langchain_text_splitters/markdown.py

@@ -107,7 +107,9 @@ class MarkdownHeaderTextSplitter:
 
         for line in lines:
             stripped_line = line.strip()
-
+            # Remove all non-printable characters from the string, keeping only visible
+            # text.
+            stripped_line = "".join(filter(str.isprintable, stripped_line))
             if not in_code_block:
                 # Exclude inline code spans
                 if stripped_line.startswith("```") and stripped_line.count("```") == 1:

libs/text-splitters/tests/unit_tests/test_text_splitters.py

@@ -1220,6 +1220,38 @@ def test_md_header_text_splitter_fenced_code_block_interleaved(
     assert output == expected_output
 
 
+@pytest.mark.parametrize("characters", ["\ufeff"])
+def test_md_header_text_splitter_with_invisible_characters(characters: str) -> None:
+    """Test markdown splitter by header: Fenced code block."""
+
+    markdown_document = (
+        f"{characters}# Foo\n\n" "foo()\n" f"{characters}## Bar\n\n" "bar()"
+    )
+
+    headers_to_split_on = [
+        ("#", "Header 1"),
+        ("##", "Header 2"),
+    ]
+
+    markdown_splitter = MarkdownHeaderTextSplitter(
+        headers_to_split_on=headers_to_split_on,
+    )
+    output = markdown_splitter.split_text(markdown_document)
+
+    expected_output = [
+        Document(
+            page_content="foo()",
+            metadata={"Header 1": "Foo"},
+        ),
+        Document(
+            page_content="bar()",
+            metadata={"Header 1": "Foo", "Header 2": "Bar"},
+        ),
+    ]
+
+    assert output == expected_output
+
+
 def test_solidity_code_splitter() -> None:
     splitter = RecursiveCharacterTextSplitter.from_language(
         Language.SOL, chunk_size=CHUNK_SIZE, chunk_overlap=0