fix: unblock MpReceiver.close() to prevent pytest hang on shutdown

MpReceiver.close() only set the closed flag and closed the buffer pipe,
but did not unblock a thread stuck on queue.get() in receive_async().
This caused abandoned threads (from abandon_on_cancel=True) to keep the
Python process alive indefinitely after all tests passed, leading to
6-hour CI timeouts on aarch64-darwin.

Send an _MpEndOfStream sentinel before closing the buffer, mirroring
what MpSender.close() already does, so the blocked get() returns and
the thread can exit cleanly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/exo/utils/channels.py

@@ -204,6 +204,10 @@ class MpReceiver[T]:
     def close(self) -> None:
         if not self._state.closed.is_set():
             self._state.closed.set()
+        try:  # noqa: SIM105
+            self._state.buffer.put_nowait(_MpEndOfStream())
+        except Exception:
+            pass
         self._state.buffer.close()
 
     # == unique to Mp channels ==