[Serve] Set num_cpus to 0 in run_graph() task

[shrekris-anyscale]

Why are these changes needed?

The Serve REST API launches an intermediate Ray task when running a deployment graph. #25861 revealed that this task currently consumes CPUs, reducing the available resources on the Ray cluster without notifying the user. This change sets the task's num_cpus to 0, similar to the ServeController actor.

Related issue number

N/A

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
    • This change adds test_run_graph_task_uses_zero_cpus to test_standalone.py.

[brucez-anyscale]

lgtm