Fix string termination in unit test

Add back the missing quote to properly close the triple-quoted string.
The expected string now has the correct number of closing quotes:
- One quote to close the inner string
- Three quotes to close the triple-quoted string

tests/unit/test_app.py

@@ -520,7 +520,7 @@ And some more text after.
 
         result = "\n".join(code_block_lines)
         expected = """def test_function():
-    return "Hello, World!""""
+    return "Hello, World!\""""
 
         self.assertEqual(result, expected)