Cosmos Driver: Restore collection macOS machine diagnostics

In #3951 I removed the collection of machine diagnostics (CPU/memory usage) on macOS because it was causing a SIGBUS crash (Copilot suspected mis-alignment of the struct but I wasn't able to resolve it so I just removed it). While macOS isn't a key deployment scenario, it would be good to have full diagnostics on macOS, so we should look at restoring it and fixing the bug in it.

Start with the changes in #3951 and trace the Cosmos driver's macOS machine-diagnostics path, focusing on the removed CPU and memory collection and the SIGBUS crash. Reproduce or investigate the macOS failure, then verify that full diagnostics are restored without triggering the crash.

Written by the indexing model from the issue text.