fix(test): use explicit import for anyio.lowlevel.checkpoint

Import checkpoint directly from anyio.lowlevel to fix pyright
reportAttributeAccessIssue on the lazy submodule.

Author: andylim-duo

tests/server/test_multi_tenancy_session.py

@@ -4,6 +4,7 @@
 
 import anyio
 import pytest
+from anyio.lowlevel import checkpoint
 
 from mcp import Client
 from mcp.server import Server
@@ -281,7 +282,7 @@ async def simulate_request(tenant_id: str, request_key: str) -> None:
         try:
             # Yield control to allow other tasks to run. This is the critical
             # point where context leakage could occur if isolation is broken.
-            await anyio.lowlevel.checkpoint()
+            await checkpoint()
 
             # Read back the tenant_id - should still be our tenant, not the other
             results[request_key] = tenant_id_var.get()