Fix syntax error in JSON error handler and ensure error details are escaped safely

Correct a syntax error in the exception handling of finalize_and_doc() that produced invalid Python code in the f-string. The code now uses json.dumps to build a proper JSON error response, eliminating the unexpected character after the line continuation character.

app.py

@@ -490,10 +490,12 @@ def finalize_and_doc(chat_history, chat_pairs, sources, plan, chat_key):
     try:
         json_string = plan_course(chat_history, sources)
     except Exception as e:
-        json_string = (
-            "{\n  \"error\": \"An error occurred while generating the course outline.\",\n"
-            f"  \"details\": \"{str(e).replace('"', '\\"')}\"\n}}"
-        )
+        # Construct a minimal JSON error message safely using json.dumps to escape characters
+        error_obj = {
+            "error": "An error occurred while generating the course outline.",
+            "details": str(e),
+        }
+        json_string = json.dumps(error_obj, indent=2)
     # Attempt to parse the JSON to ensure it is valid; if it fails, wrap as raw string
     try:
         parsed = json.loads(json_string)