fix: _sanitize_export_filename crashes on a non-string session name (#1607)

tests/test_session_export_filename.py

@@ -0,0 +1,15 @@
+"""Regression: _sanitize_export_filename must tolerate a non-string name.
+
+It did `name = name or ""` then `re.sub(..., name)`. A non-string name (e.g. an
+int session name) is truthy, so re.sub raised TypeError. Coerce non-strings.
+"""
+from routes.session_routes import _sanitize_export_filename
+
+
+def test_non_string_name_does_not_crash():
+    assert _sanitize_export_filename(12345) == ""
+    assert _sanitize_export_filename(None) == ""
+
+
+def test_valid_name_sanitized():
+    assert _sanitize_export_filename("a/b?c.txt") == "a_b_c.txt"