Fix ResourceWarning by properly closing file handles in DummyProcess

The DummyProcess class wasn't closing file handles in its __aexit__ method,
causing ResourceWarning errors in CI tests on Windows. This commit adds
proper cleanup to close both the async stream wrappers and underlying raw
file handles when the process context manager exits.

This prevents file descriptor leaks and eliminates the ResourceWarning
errors seen in pytest output on Windows CI.

Author: felixweinberger

src/mcp/client/stdio/win32.py

@@ -79,6 +79,18 @@ async def __aexit__(
         self.popen.terminate()
         await to_thread.run_sync(self.popen.wait)
 
+        # Close the file handles to prevent ResourceWarning
+        if self.stdin:
+            await self.stdin.aclose()
+        if self.stdout:
+            await self.stdout.aclose()
+        if self.stdin_raw:
+            self.stdin_raw.close()
+        if self.stdout_raw:
+            self.stdout_raw.close()
+        if self.stderr:
+            self.stderr.close()
+
     async def wait(self):
         """Async wait for process completion."""
         return await to_thread.run_sync(self.popen.wait)