Fix escaped quote in unit test expected string

Remove stray backslash from expected multiline string in test_find_most_recent_code_block.
The expected string now correctly matches the extracted code block content:
- def test_function():
-     return "Hello, World\!"

This fixes the test assertion to match the actual extracted content exactly.

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)