fix: explicitly delete error references to prevent memory leaks

Add explicit "del error" statement to prevent reference cycles between
exception objects and local variables. This helps the garbage collector
promptly reclaim memory after request processing, which is especially
important in high-traffic web applications where exception objects with
traceback information can consume significant memory.

src/flask/app.py

@@ -1525,6 +1525,7 @@ class Flask(App):
                 error = None
 
             ctx.pop(error)
+            del error
 
     def __call__(
         self, environ: WSGIEnvironment, start_response: StartResponse