fix: markdown tables drop empty cells and misalign columns (#1164)

* refactor: extract splitTableRow helper for markdown tables

* fix: keep empty interior cells in markdown tables to preserve columns

* test: splitTableRow keeps empty interior cells

tests/test_markdown_table_row_js.py

@@ -0,0 +1,47 @@
+"""Pin the pure splitTableRow helper (static/js/markdown/tableRow.js).
+
+Driven through `node --input-type=module` (same approach as test_compare_js.py);
+skips when `node` is not installed.
+
+Regression: the old split filtered out every empty cell, so an intentionally
+empty interior cell ("| a |  | c |") collapsed the row to 2 columns and
+misaligned it with the header.
+"""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_HELPER = _REPO / "static" / "js" / "markdown" / "tableRow.js"
+_HAS_NODE = shutil.which("node") is not None
+
+
+def _split(row: str):
+    js = f"""
+    import {{ splitTableRow }} from '{_HELPER.as_posix()}';
+    console.log(JSON.stringify(splitTableRow({json.dumps(row)})));
+    """
+    proc = subprocess.run(
+        ["node", "--input-type=module"],
+        input=js, capture_output=True, text=True, cwd=str(_REPO), timeout=30,
+    )
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout.strip())
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_keeps_empty_interior_cell():
+    assert _split("| a |  | c |") == ["a", "", "c"]
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_rows_without_outer_pipes():
+    assert _split("a | b | c") == ["a", "b", "c"]
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_header_row_unaffected():
+    assert _split("| h1 | h2 | h3 |") == ["h1", "h2", "h3"]