fix: is_markitdown_format crashes on a non-string path (#1618)

tests/test_markitdown_format_nonstring.py

@@ -0,0 +1,16 @@
+"""Regression: is_markitdown_format must tolerate a non-string path.
+
+It did `os.path.splitext(path)`, which raises TypeError on None / non-string.
+"""
+from src.markitdown_runtime import is_markitdown_format
+
+
+def test_non_string_returns_false():
+    assert is_markitdown_format(None) is False
+    assert is_markitdown_format(123) is False
+    assert is_markitdown_format(["a.docx"]) is False
+
+
+def test_valid_extension_detected():
+    assert is_markitdown_format("report.docx") is True
+    assert is_markitdown_format("notes.txt") is False