[core] (cgroups 20/n) Move dashboard api server's subprocesses into system cgroup

[israbbani]

For more details about the resource isolation project see #54703.

When starting the head node, move the dashboard api server's subprocesses into the system cgroup. I updated the integration test and added a helpful error message because the test will break in the future when a new dashboard module is added.

I ran the integration tests 25 times locally.

(ray2) ubuntu@devbox:~/code/ray2$ python -m pytest -s python/ray/tests/resource_isolation/test_resource_isolation_integration.py --count 25 -x
...
collecting ...
python/ray/tests/resource_isolation/test_resource_isolation_integration.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓ 25% ██▌ 2025-10-17 23:13:51,897 INFO worker.py:1833 -- Connecting to existing Ray cluster at address: 172.31.12.251:6379...
2025-10-17 23:13:51,905 INFO worker.py:2004 -- Connected to Ray cluster. View the dashboard at http://127.0.0.1:8265
python/ray/tests/resource_isolation/test_resource_isolation_integration.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓ 26% ██▋ 2025-10-17 23:13:57,592 INFO worker.py:1833 -- Connecting to existing Ray cluster at address: 172.31.12.251:6379...
2025-10-17 23:13:57,598 INFO worker.py:2004 -- Connected to Ray cluster. View the dashboard at http://127.0.0.1:8265
python/ray/tests/resource_isolation/test_resource_isolation_integration.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓ 98% █████████▊2025-10-17 23:19:45,417 INFO worker.py:2004 -- Started a local Ray instance. View the dashboard at http://127.0.0.1:8265
python/ray/tests/resource_isolation/test_resource_isolation_integration.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓ 99% █████████▉2025-10-17 23:19:50,194 INFO worker.py:2004 -- Started a local Ray instance. View the dashboard at http://127.0.0.1:8265
python/ray/tests/resource_isolation/test_resource_isolation_integration.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓
100% ██████████
Results (366.41s):
100 passed