install-arc verification fails on azcmagent v1.62+ (gcarcservice renamed to gcad)

## Summary

`install-arc` verification fails on hosts running **Azure Connected Machine Agent (`azcmagent`) v1.62+** because the service `gcarcservice` was renamed to `gcad` upstream, but `pkg/arc/consts.go` still hardcodes the old name. The Arc connection itself succeeds (`azcmagent show` reports `Agent Status: Connected`); only flex-node's verification check fails.

## Environment

- AKSFlexNode: `v0.1.0` (and `main` at time of writing — same code)
- Host: Raspberry Pi 5, Ubuntu 24.04.4 LTS, arm64
- Azure Connected Machine Agent: `1.62.03365.964`
- AKS managed cluster: K8s 1.34.7, AAD + Azure RBAC enabled, westus2

## Reproduction

1. Onboard a fresh Ubuntu 24.04 host with the latest `azcmagent` (`curl -sSL https://aka.ms/azcmagent-linux | bash` pulls v1.62+ today).
2. Pre-grant the resulting Arc identity Reader on subscription + User Access Administrator on subscription (so `assignRBACRoles` can succeed).
3. Run `aks-flex-node start --config /etc/aks-flex-node/config.json`.

## Observed

`install-arc` reports all four RBAC roles assigned, RBAC propagated, then immediately:

```
level=ERROR msg=failed task=install-arc duration=23.4s status=failed
error="arc installation completed but verification failed"
```

Meanwhile on the host:

```
$ azcmagent show | grep "Agent Status"
Agent Status : Connected

$ systemctl is-active himdsd gcad extd
active
active
active

$ systemctl is-active gcarcservice
inactive # service does not exist under this name on azcmagent v1.62+
```

## Root cause

`pkg/arc/consts.go`:

```go
// Arc services that may be present (not all are guaranteed to exist on every installation)
arcServices = []string{"himdsd", "gcarcservice", "extd"}
```

…combined with `pkg/arc/helpers.go::isArcServicesRunning`, which requires **all** services to be active:

```go
for _, service := range arcServices {
if !utilexec.IsServiceActive(ctx, logger, service) {
return false
}
}
```

The comment says *"not all are guaranteed to exist"*, but the implementation is strict-AND, so a single rename upstream breaks the verification gate.

`arcServiceFiles` in the same file (used for uninstall/cleanup) has the same stale path: `/lib/systemd/system/gcarcservice.service`.

## Workaround

```bash
sudo ln -s /lib/systemd/system/gcad.service /etc/systemd/system/gcarcservice.service
sudo systemctl daemon-reload
```

After this symlink, `systemctl is-active gcarcservice` returns `active` and `install-arc` verification passes.

## Suggested fix options

1. **Trust `azcmagent show`.** `isCompleted` already parses `Agent Status: Connected`. Make `isArcServicesRunning` a soft check (log + warn) and let the `azcmagent show` line be authoritative. This is the most robust — service names will keep drifting as the Arc agent evolves.
2. **Accept either name.** Replace `"gcarcservice"` with `[]string{"gcarcservice", "gcad"}` and require at least one to be active.
3. **Match the comment.** Make the loop tolerant: warn on missing services rather than fail. Combined with the `azcmagent show` parse, this is sufficient.

Also update `arcServiceFiles` for uninstall to include `gcad.service`, otherwise cleanup will leak the service file on hosts using the new agent.

## Impact

Anyone onboarding flex-node on a freshly-installed host today will hit this, since `aka.ms/azcmagent-linux` serves v1.62+. The error message *"arc installation completed but verification failed"* doesn't point at the cause and looks like an RBAC/connectivity problem, so it's easy to chase for an hour before locating it in source.

Happy to send a PR if helpful — just want to confirm which of the three approaches you prefer.