Address code review: use = None instead of del in finally blocks, improve test

Changed del frame/del arg to frame = None/arg = None in finally blocks to
avoid potential UnboundLocalError. Improved test to only get frame when needed.

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

Author: Copilot

src/debugpy/_vendored/pydevd/_pydevd_sys_monitoring/_pydevd_sys_monitoring.py

@@ -930,8 +930,8 @@ def _unwind_event(code, instruction, exc):
         # Clear frame and arg references to avoid preventing garbage collection
         # of objects referenced from the frame's locals. The arg tuple also
         # contains the traceback which may hold frame references.
-        del frame
-        del arg
+        frame = None
+        arg = None
 
 
 # fmt: off
@@ -994,8 +994,8 @@ def _raise_event(code, instruction, exc):
         # Clear frame and arg references to avoid preventing garbage collection
         # of objects referenced from the frame's locals. The arg tuple also
         # contains the traceback which may hold frame references.
-        del frame
-        del arg
+        frame = None
+        arg = None
 
 
 # fmt: off

src/debugpy/_vendored/pydevd/_pydevd_sys_monitoring/_pydevd_sys_monitoring_cython.c

@@ -936,8 +936,8 @@ cdef _unwind_event(code, instruction, exc):
         # Clear frame and arg references to avoid preventing garbage collection
         # of objects referenced from the frame's locals. The arg tuple also
         # contains the traceback which may hold frame references.
-        del frame
-        del arg
+        frame = None
+        arg = None
 
 
 # fmt: off
@@ -1000,8 +1000,8 @@ cdef _raise_event(code, instruction, exc):
         # Clear frame and arg references to avoid preventing garbage collection
         # of objects referenced from the frame's locals. The arg tuple also
         # contains the traceback which may hold frame references.
-        del frame
-        del arg
+        frame = None
+        arg = None
 
 
 # fmt: off

src/debugpy/_vendored/pydevd/_pydevd_sys_monitoring/_pydevd_sys_monitoring_cython.pyx

@@ -314,15 +314,15 @@ def __init__(self, name):
     refs = []
 
     def _start_method(code, offset):
-        # Gets frame only if needed (simulating the improved pattern).
-        frame = sys._getframe(1)
+        # Only obtain the frame when actually needed (e.g., to enable local events).
         if "test_sys_monitoring" in code.co_filename:
+            frame = sys._getframe(1)
             monitor.set_local_events(DEBUGGER_ID, code, monitor.events.PY_RETURN)
 
     def _return_method(code, offset, retval):
-        # In the fixed code, the frame is only obtained when step_cmd != -1.
-        # Here we simulate the common path where no stepping is happening.
-        pass  # No frame obtained when not needed
+        # Simulate the common path where no stepping is happening:
+        # the frame should NOT be obtained when not needed.
+        pass
 
     monitor.set_events(DEBUGGER_ID, monitor.events.PY_START)
     monitor.register_callback(DEBUGGER_ID, monitor.events.PY_START, _start_method)