bug(vm): gateway compute-driver.sock path can exceed macOS sun_path limit #5015

User Story

As a developer running the VM compute driver on macOS, I want the gateway's compute-driver socket path to fit within the OS sun_path limit so that the gateway starts no matter where I keep VM driver state.

Problem Statement

The gateway places the VM driver's control socket at {state_dir}/run/compute-driver.sock. On macOS, struct sockaddr_un.sun_path is 104 bytes (103 usable). The fixed /run/compute-driver.sock suffix takes 24 bytes, so any state_dir longer than 79 bytes makes the driver fail to bind, and the gateway does not start.

For example, with a state_dir inside a checkout:

/Users/bmahabir/src/github.com/NVIDIA/OpenShell/.cache/gateway-vm/vm-driver-state/run/compute-driver.sock
└──────────────────────────────────────── 105 bytes ───────────────────────────────────────────────┘

Path length breakdown:

/Users/bmahabir/src/github.com/NVIDIA/OpenShell/.cache/gateway-vm/vm-driver-state   (81)
/run/compute-driver.sock                                                             (24)
                                                                         total = 105 bytes
                                                           macOS usable limit = 103 bytes
                                                                    over by 2 bytes

#3452 hit the same limit for the per-sandbox control.sock and ssh.sock. #3544 fixed those by moving them to a short, randomly named directory under /tmp, but explicitly left run/compute-driver.sock unchanged. This is the remaining socket.

Impact / Why This Matters

The gateway fails at startup with path must be shorter than SUN_LEN, which doesn't name the path or the setting that controls it (see #4178).

The current workaround is to set a short state_dir such as /tmp/os-vm. This is non-obvious and moves persistent VM state (overlays, image cache, logs) into /tmp, which is cleared on reboot. The repo's own dev scripts (tasks/scripts/gateway-vm.sh, e2e/rust/e2e-vm.sh) already put state under /tmp for this reason.

Linux is less affected (108-byte limit) but not immune with deep state directories.

Acceptance Criteria

• The gateway starts with the VM driver on macOS for any reasonable state_dir, following the same approach as fix(vm): relocate per-sandbox Unix sockets to /tmp to fit macOS sun_path #3544
• VM state still persists under the configured state_dir
• The socket and its directory are cleaned up when the gateway shuts down
• If the socket path still can't fit, the error names the path, its length, and the limit (resolves bug(vm-driver): driver startup fails with a bare "path must be shorter than SUN_LEN" and does not name the path #4178)

Reproduction Steps

1. On macOS, set state_dir under [openshell.drivers.vm] to a path longer than 79 bytes
2. Start the gateway with compute_driver = "vm"
3. Observe the driver exits before the gateway becomes ready

Environment

• OpenShell: development build (current main branch)
• OS: macOS (Apple Silicon) — sun_path is 104 bytes
• Runtime: VM compute driver (libkrun)

Logs

Same output as reported in #4178:

Error:   × path must be shorter than SUN_LEN
Error:   × execution error: vm compute driver exited before becoming ready with
  │ status exit status: 1