fix: ICS export doesn't escape commas/semicolons in event fields (#1161)

* fix: escape SUMMARY/LOCATION per RFC 5545 in ICS export

* fix: escape commas/semicolons in ICS DESCRIPTION, not just newlines

* test: ICS export escapes commas, semicolons, backslashes, newlines

tests/test_ics_escape.py

@@ -0,0 +1,25 @@
+"""Tests for iCalendar TEXT escaping in calendar export (RFC 5545 §3.3.11)."""
+from tests.test_null_owner_gates import _import_calendar_helpers
+
+
+def _esc():
+    return _import_calendar_helpers()._ics_escape
+
+
+def test_escapes_comma_and_semicolon():
+    # Regression: SUMMARY/LOCATION escaped nothing, so a comma/semicolon
+    # (structural in iCal TEXT values) corrupted the field in other clients.
+    assert _esc()("Lunch, dinner; meeting") == "Lunch\\, dinner\\; meeting"
+
+
+def test_escapes_backslash_first():
+    assert _esc()("path C:\\tmp") == "path C:\\\\tmp"
+
+
+def test_newlines_become_literal_backslash_n():
+    assert _esc()("line1\nline2\r\nline3") == "line1\\nline2\\nline3"
+
+
+def test_empty_and_none_safe():
+    assert _esc()("") == ""
+    assert _esc()(None) == ""