Fix Windows test hanging due to double process termination

The FallbackProcess.__aexit__ method was automatically terminating
processes on context exit, which conflicted with stdio_client's
explicit cleanup sequence. This caused the test_stdio_client_universal_cleanup
test to hang on Windows.

The fix removes automatic termination from FallbackProcess.__aexit__,
allowing stdio_client to handle the MCP spec-compliant shutdown sequence:
1. Close stdin and wait for graceful exit
2. Send terminate signal if needed
3. Force kill as last resort

This prevents the double termination attempt that was causing hangs.

Reported-by: fweinberger

Author: felixweinberger

src/mcp/client/stdio/win32.py

@@ -73,10 +73,11 @@ async def __aexit__(
         exc_val: BaseException | None,
         exc_tb: object | None,
     ) -> None:
-        """Terminate and wait on process exit inside a thread."""
-        self.popen.terminate()
-        await to_thread.run_sync(self.popen.wait)
-
+        """Clean up streams without terminating the process.
+        
+        The process termination is handled by stdio_client's cleanup sequence
+        which implements the MCP spec-compliant shutdown flow.
+        """
         # Close the file handles to prevent ResourceWarning
         if self.stdin:
             await self.stdin.aclose()