Fix Windows CI: move GetThreadContext/SetThreadContext to correct import path and fix HANDLE type

- GetThreadContext and SetThreadContext are in Win32::System::Diagnostics::Debug, not Win32::System::Threading
- HANDLE is *mut c_void on Windows, not usize; use std::ptr::null_mut() for initialization

Agent-Logs-Url: https://github.com/acl-dev/open-coroutine/sessions/5486070d-5dbe-4036-81a6-18cde48b0725

Co-authored-by: loongs-zhang <38336731+loongs-zhang@users.noreply.github.com>

Author: Copilot

core/src/monitor.rs

@@ -24,9 +24,11 @@ cfg_if::cfg_if! {
         };
         use windows_sys::Win32::System::Threading::{
             GetCurrentProcess, GetCurrentThread,
-            SuspendThread, ResumeThread, GetThreadContext, SetThreadContext,
+            SuspendThread, ResumeThread,
+        };
+        use windows_sys::Win32::System::Diagnostics::Debug::{
+            CONTEXT, GetThreadContext, SetThreadContext,
         };
-        use windows_sys::Win32::System::Diagnostics::Debug::CONTEXT;
     }
 }
 
@@ -490,7 +492,7 @@ impl Monitor {
                 };
             } else if #[cfg(windows)] {
                 let node = unsafe {
-                    let mut real_handle: HANDLE = 0;
+                    let mut real_handle: HANDLE = std::ptr::null_mut();
                     let result = DuplicateHandle(
                         GetCurrentProcess(),
                         GetCurrentThread(),