Use getattr for safe singleThread attribute access in on_continue_request

Most DAP clients omit the optional singleThread field entirely. Using
getattr(arguments, 'singleThread', False) is more defensive than direct
attribute access, guarding against any non-standard arguments objects.

Co-authored-by: rchiodo <19672699+rchiodo@users.noreply.github.com>

Author: Copilot

src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_process_net_command_json.py

@@ -564,7 +564,9 @@ def on_continue_request(self, py_db, request):
         # unless singleThread is explicitly true (and the capability
         # supportsSingleThreadExecutionRequests is advertised). Only use the
         # specific threadId when singleThread is set; otherwise resume all.
-        single_thread = arguments.singleThread
+        # Use getattr with a default of False since most DAP clients omit this
+        # optional field entirely.
+        single_thread = getattr(arguments, "singleThread", False)
         if not single_thread or py_db.multi_threads_single_notification:
             thread_id = "*"