test: add nine-backslash escape test for prompt placeholders

tests/test_prompt.py

@@ -43,6 +43,15 @@ class TestSubstitutePlaceholders:
         result = substitute_placeholders("Hello \\\\\\{name.txt}", project_dir)
         assert result == "Hello \\{name.txt}"
 
+    def test_nine_backslashes_before_placeholder(self, project_dir: Path) -> None:
+        """Nine backslashes (odd) → 4 literal backslashes + literal {file.txt}."""
+        _ = (project_dir / "file.txt").write_text("content")
+        # 9 backslashes before {file.txt}: odd → escaped, halve to 4
+        result = substitute_placeholders(
+            "\\\\\\\\\\\\\\\\\\" + "{file.txt}", project_dir
+        )
+        assert result == "\\\\\\\\{file.txt}"
+
     def test_placeholder_at_start(self, project_dir: Path) -> None:
         _ = (project_dir / "greeting.txt").write_text("Hi there")
         result = substitute_placeholders("{greeting.txt}!", project_dir)