Add returncode property to FallbackProcess; simplify returncode guard

FallbackProcess stands in for anyio.abc.Process on Windows with
SelectorEventLoop, but was missing the returncode property that Process
declares abstractly. This meant getattr(proc, "returncode", None) in
_terminate_and_reap always returned the default None for FallbackProcess,
so the double-terminate guard never fired (the docstring's 'Idempotent'
claim was false for half the declared union type).

Practical impact was near-zero — FallbackProcess is only active under
SelectorEventLoop (not the default since Python 3.8, not used in CI),
and terminate_windows_process_tree swallows all exceptions on the second
call. But the fix is trivial and brings FallbackProcess closer to the
interface it's supposed to implement.

With returncode present on both union members, the getattr can become a
direct attribute access, which also satisfies pyright without defensive
indirection.

Github-Issue: #1775

Author: maxisbey

src/mcp/os/win32/utilities.py

@@ -123,6 +123,11 @@ def pid(self) -> int:
         """Return the process ID."""
         return self.popen.pid
 
+    @property
+    def returncode(self) -> int | None:
+        """Return the exit code, or ``None`` if the process has not yet terminated."""
+        return self.popen.returncode
+
 
 # ------------------------
 # Updated function

tests/client/test_stdio.py

@@ -335,7 +335,7 @@ async def _terminate_and_reap(proc: anyio.abc.Process | FallbackProcess) -> None
     safety net. Bounded by ``move_on_after`` to prevent hangs.
     """
     with anyio.move_on_after(5.0):
-        if getattr(proc, "returncode", None) is None:
+        if proc.returncode is None:
             await _terminate_process_tree(proc)
         await proc.wait()
         assert proc.stdin is not None