Add test_stdio_cm for stdin_writer hanging

Author: LionNatsu

tests/client/test_stdio_cm.py

@@ -0,0 +1,18 @@
+import pytest
+
+from mcp import StdioServerParameters
+from mcp.client.stdio import stdio_client
+
+MCP_SERVER = {
+    "command": "uvx",
+    "args": ["mcp-server-fetch"],
+}
+
+
+@pytest.mark.anyio
+async def test_context_manager_exiting():
+    async with stdio_client(StdioServerParameters(**MCP_SERVER)) as (
+        read_stream,
+        write_stream,
+    ):
+        pass