Mark post-shielded-cancel assertions as lax-no-cover for 3.11

The exit-stack-hangs test passes on all Python versions, but
coverage.py on 3.11 misreports the assertions after the shielded
move_on_after cancellation as unhit (the tracer in the test frame is
disrupted by the cancel inside the request task). lax no cover is the
sanctioned exclusion for lines covered on some versions but not others.

Claude-Session: https://claude.ai/code/session_017S3aJaxEHeMvftp6whnHWK

Author: maxisbey

tests/server/test_experimental_streamable_http_modern.py

@@ -183,7 +183,8 @@ async def list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams |
     with anyio.fail_after(5), caplog.at_level(logging.WARNING, logger=modern.__name__):
         async with _asgi_client(Server("test", on_list_tools=list_tools)) as http:
             response = await http.post("/mcp", json=_list_tools_body(), headers={"content-type": "application/json"})
-
-    assert response.status_code == 200
-    assert response.json()["result"]["tools"] == []
-    assert "abandoning remaining callbacks" in caplog.text
+    # coverage.py on Python 3.11 misreports the lines below as unhit (the test passes there);
+    # the shielded-cancel path inside the request task disrupts the tracer in this frame.
+    assert response.status_code == 200  # pragma: lax no cover
+    assert response.json()["result"]["tools"] == []  # pragma: lax no cover
+    assert "abandoning remaining callbacks" in caplog.text  # pragma: lax no cover