fix: use MagicMock for client to support async context manager

ctx.client needs to be MagicMock (not AsyncMock) because stream()
returns an async context manager, not a coroutine.

Author: zsxh1990

tests/client/test_non2xx_status.py

@@ -26,7 +26,7 @@ def _make_request_context(self, request_id: str = "test-123") -> MagicMock:
         ctx.session_message.message = MagicMock(spec=JSONRPCRequest)
         ctx.session_message.message.id = request_id
         ctx.read_stream_writer = AsyncMock()
-        ctx.client = AsyncMock()
+        ctx.client = MagicMock()  # Use MagicMock for client (not AsyncMock)
         ctx.metadata = None
         return ctx