fix: visual_report markdown helpers crash on a non-string input (#1633)

tests/test_visual_report_nonstring.py

@@ -0,0 +1,18 @@
+"""Regression: visual_report markdown helpers must tolerate a non-string.
+
+_autolink_urls did `re.sub(..., md_text)` and _extract_headings did
+`re.finditer(..., md_text)`; a None/non-string raised TypeError. They now
+return the input / [] respectively.
+"""
+from src.visual_report import _autolink_urls, _extract_headings
+
+
+def test_non_string_does_not_crash():
+    assert _autolink_urls(None) is None
+    assert _extract_headings(None) == []
+    assert _extract_headings(123) == []
+
+
+def test_valid_markdown_unchanged():
+    assert "](https://x.com)" in _autolink_urls("see https://x.com")
+    assert _extract_headings("## Title")[0]["text"] == "Title"