fix: CGroupLimits should detect memory limits in sub-cgroups

[praveen-influx]

closes: #1610

Integration test ran manually

• Run test_cgroup_limits_with_memory_constraint with 512M bound - passes as the test expects memory to be reported as 512M

❯ systemd-run --user --scope -p MemoryMax=512M cargo test --test cgroup_integration -- --ignored --nocapture
Running as unit: run-p1081902-i5275437.scope; invocation ID: fa8f6fa08f334494bf03d2ef7ca23214
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.01s
     Running tests/cgroup_integration.rs (target/debug/deps/cgroup_integration-e2b02b9d93054168)

running 1 test
CGroup limits detected: CGroupLimits { total_memory: 536870912, free_memory: 525611008, free_swap: 4294963200, rss: 9125888 }
test test_cgroup_limits_with_memory_constraint ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

• Run test_cgroup_limits_with_memory_constraint with 1024M bound - fails the test

❯ systemd-run --user --scope -p MemoryMax=1024M cargo test --test cgroup_integration -- --ignored --nocapture
Running as unit: run-p1081827-i5275362.scope; invocation ID: 79b08c4bbb3c44a888a7e3505ee9939a
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.01s
     Running tests/cgroup_integration.rs (target/debug/deps/cgroup_integration-e2b02b9d93054168)

running 1 test

thread 'test_cgroup_limits_with_memory_constraint' (1081844) panicked at tests/cgroup_integration.rs:17:9:
Expected 512MB limit, got 1073741824 bytes
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test test_cgroup_limits_with_memory_constraint ... FAILED

failures:

failures:
    test_cgroup_limits_with_memory_constraint

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--test cgroup_integration`

[praveen-influx]

@GuillaumeGomez - any idea why this check is failing, https://github.com/GuillaumeGomez/sysinfo/actions/runs/20441080932/job/58734423014?pr=1612, it looks unrelated to my changes (seems like a windows error) but I wanted to check before I started debugging the failure.