[core] (cgroups 19/n) Allow fractions when getting the number of CPUs to calculate weights

[israbbani]

This PR stacks on #57776.

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

When Ray calculates the number of cpus available on the machine, it checks to see if it's running in a container. However, it truncates the number of cpus.

In this PR,

• If the number of CPUs on the machine is <= DEFAULT_MIN_SYSTEM_RESERVED_CPU_CORES, then raise a ValueError. Previously, this was < DEFAULT_MIN_SYSTEM_RESERVED_CPU_CORES.
• Return fractional CPUs from ray._private.utils.get_num_cpus if an optional parameter is set to True.

[israbbani]

Tested on Anyscale w/ a 2 core machine. Works with default parameters now.

lscpu | grep "CPU(s)"
CPU(s):                                  2

cat /sys/fs/cgroup/ray-node_e06784dc2316943d0918f0257d1d2cb24d24605ea9022d745cb23fe4/user/cpu.weight
4445

cat /sys/fs/cgroup/ray-node_e06784dc2316943d0918f0257d1d2cb24d24605ea9022d745cb23fe4/system/cpu.weight
5555

From the logs

(base) ray@ip-10-0-251-150:~/default$ grep "CgroupManager" /tmp/ray/session_latest/logs/raylet.out
{"asctime":"2025-10-16 12:49:53,974","levelname":"I","message":"Initializing CgroupManager at base cgroup at '/sys/fs/cgroup'. Ray's cgroup hierarchy will under the node cgroup at '/sys/fs/cgroup/ray-node_e06784dc2316943d0918f0257d1d2cb24d24605ea9022d745cb23fe4' with [memory, cpu] controllers enabled. The system cgroup at '/sys/fs/cgroup/ray-node_e06784dc2316943d0918f0257d1d2cb24d24605ea9022d745cb23fe4/system' will have [memory] controllers enabled with [cpu.weight=5555, memory.min=5946149682] constraints. The user cgroup '/sys/fs/cgroup/ray-node_e06784dc2316943d0918f0257d1d2cb24d24605ea9022d745cb23fe4/user' will have no controllers enabled with [cpu.weight=4445] constraints. The user cgroup will contain the [/sys/fs/cgroup/ray-node_e06784dc2316943d0918f0257d1d2cb24d24605ea9022d745cb23fe4/user/workers, /sys/fs/cgroup/ray-node_e06784dc2316943d0918f0257d1d2cb24d24605ea9022d745cb23fe4/user/non-ray] cgroups.","component":"raylet","filename":"cgroup_manager.cc","lineno":212}

[edoakes]

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[edoakes]

should leave a comment for why we don't truncate