Add tests for str() + Content() fix

tests/test_content.py

@@ -136,6 +136,14 @@ def test_add() -> None:
     assert content.spans == [Span(0, 3, "red"), Span(4, 7, "blue")]
     assert content.cell_length == 7
 
+def test_radd() -> NOne
+    """Test reverse addition."""
+    assert "foo" + Content("bar") == Content("foobar")
+
+    # Test spans after addition
+    content = "foo " + Content.styled("bar", "blue")
+    assert str(content) == "foo bar"
+    assert content.spans == [Span(4, 7, "blue")]
 
 def test_from_markup():
     """Test simple parsing of content markup."""